[Security Solution][Serverless Quality Gate] Restoring overrides func…

…tionality (#196536)

## Summary

Due to a bug introduced in a previous PR, the `override` value was never
using the `process.env.KIBANA_MKI_IMAGE_COMMIT` environment variable.

The reason is that in the command line arguments argparse, commit value
had the default of `''` so the commit was never null or undefined.

```
    .option('commit', {
          alias: 'c',
          type: 'string',
          default: '',
        })
```

Restored the check to see if the string is also empty.

(cherry picked from commit cbe5d9a)

x-pack/plugins/security_solution/scripts/run_cypress/project_handler/cloud_project_handler.ts

@@ -44,7 +44,7 @@ export class CloudHandler extends ProjectHandler {
     // no kibana image override. The tests will be executed against the commit which is already promoted to QA.
     const qualityGate =
       process.env.KIBANA_MKI_QUALITY_GATE && process.env.KIBANA_MKI_QUALITY_GATE === '1';
-    const override = commit ?? process.env.KIBANA_MKI_IMAGE_COMMIT;
+    const override = commit && commit !== '' ? commit : process.env.KIBANA_MKI_IMAGE_COMMIT;
     if (override && !qualityGate) {
       const kibanaOverrideImage = `${override?.substring(0, 12)}`;
       this.log.info(`Kibana Image Commit under test: ${process.env.KIBANA_MKI_IMAGE_COMMIT}!`);

x-pack/plugins/security_solution/scripts/run_cypress/project_handler/proxy_project_handler.ts

@@ -44,7 +44,7 @@ export class ProxyHandler extends ProjectHandler {
     // no kibana image override. The tests will be executed against the commit which is already promoted to QA.
     const qualityGate =
       process.env.KIBANA_MKI_QUALITY_GATE && process.env.KIBANA_MKI_QUALITY_GATE === '1';
-    const override = commit ?? process.env.KIBANA_MKI_IMAGE_COMMIT;
+    const override = commit && commit !== '' ? commit : process.env.KIBANA_MKI_IMAGE_COMMIT;
     if (override && !qualityGate) {
       const kibanaOverrideImage = `${override?.substring(0, 12)}`;
       this.log.info(`Kibana Image Commit under test: ${process.env.KIBANA_MKI_IMAGE_COMMIT}!`);