Allow empty spaces un Gsub processor (#197815)

Closes [#191920](#191920)

## Summary
In the pattern field of the GSUB processor, we were validating that the
field is not empty. But for this case we must allow spaces as values. To
fix this, I've added a second parameter `trimString` to the `emptyField`
validator. If the parameter is set to true by default so the behavior of
the validator doesn't change in all the fields that already use it. But
when the parameter is set to false, as in this case, the entry value
string is not trimmed, so it only validate to false if the field is
actually empty. I also added some unit tests to make sure that this new
condition doesn't break the validator.

https://github.com/user-attachments/assets/049f7424-00c6-4bb2-ab89-2ce158ac4c4e

#### Notes
In the video it can be seen that a banner flashes when saving. It is a
bug that already happened before this one. I've opened [an
issue](#197810) to address it.

(cherry picked from commit 2f7f8e3)

src/plugins/es_ui_shared/static/forms/helpers/field_validators/empty_field.test.ts

@@ -0,0 +1,48 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import { ValidationFuncArg } from '../../hook_form_lib';
+import { emptyField } from './empty_field';
+
+describe('emptyField', () => {
+  const message = 'test error message';
+  const code = 'ERR_FIELD_MISSING';
+  const path = 'path';
+
+  const validator = (value: string | any[], trimString?: boolean) =>
+    emptyField(message, trimString)({ value, path } as ValidationFuncArg<any, any>);
+
+  test('should return Validation function if value is an empty string and trimString is true', () => {
+    expect(validator('')).toMatchObject({ message, code, path });
+  });
+
+  test('should return Validation function if value is an empty string and trimString is false', () => {
+    expect(validator('', false)).toMatchObject({ message, code, path });
+  });
+
+  test('should return Validation function if value is a space and trimString is true', () => {
+    expect(validator(' ')).toMatchObject({ message, code, path });
+  });
+
+  test('should return undefined if value is a space and trimString is false', () => {
+    expect(validator(' ', false)).toBeUndefined();
+  });
+
+  test('should return undefined if value is a string and is not empty', () => {
+    expect(validator('not Empty')).toBeUndefined();
+  });
+
+  test('should return undefined if value an array and is not empty', () => {
+    expect(validator(['not Empty'])).toBeUndefined();
+  });
+
+  test('should return undefined if value an array and is empty', () => {
+    expect(validator([])).toMatchObject({ message, code, path });
+  });
+});

src/plugins/es_ui_shared/static/forms/helpers/field_validators/empty_field.ts

@@ -13,12 +13,14 @@ import { isEmptyArray } from '../../../validators/array';
 import { ERROR_CODE } from './types';
 
 export const emptyField =
-  (message: string) =>
+  (message: string, trimString: boolean = true) =>
   (...args: Parameters<ValidationFunc>): ReturnType<ValidationFunc<any, ERROR_CODE>> => {
     const [{ value, path }] = args;
 
     if (typeof value === 'string') {
-      return isEmptyString(value) ? { code: 'ERR_FIELD_MISSING', path, message } : undefined;
+      return isEmptyString(value, trimString)
+        ? { code: 'ERR_FIELD_MISSING', path, message }
+        : undefined;
     }
 
     if (Array.isArray(value)) {

src/plugins/es_ui_shared/static/validators/string/is_empty.ts

@@ -7,4 +7,5 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-export const isEmptyString = (value: string) => value.trim() === '';
+export const isEmptyString = (value: string, trimString: boolean = true) =>
+  (trimString ? value.trim() : value) === '';

x-pack/plugins/ingest_pipelines/public/application/components/pipeline_editor/components/processor_form/processors/gsub.tsx

@@ -37,7 +37,8 @@ const fieldsConfig: FieldsConfig = {
         validator: emptyField(
           i18n.translate('xpack.ingestPipelines.pipelineEditor.gsubForm.patternRequiredError', {
             defaultMessage: 'A value is required.',
-          })
+          }),
+          false
         ),
       },
       {