chore: omit null defaultValue in bpmetadata

cli/bpmetadata/int-test/goldens/golden-metadata.yaml

@@ -151,7 +151,6 @@ spec:
       - name: names
         description: Bucket name suffixes.
         varType: list(string)
-        defaultValue: null
         required: true
       - name: prefix
         description: Prefix used to generate the bucket name.
@@ -160,7 +159,6 @@ spec:
       - name: project_id
         description: Bucket project id.
         varType: string
-        defaultValue: null
         required: true
       - name: public_access_prevention
         description: Prevents public access to a bucket. Acceptable values are inherited or enforced. If inherited, the bucket uses public access prevention, only if the bucket is subject to the public access prevention organization policy constraint.

cli/bpmetadata/tfconfig.go

@@ -231,12 +231,12 @@ func getBlueprintVariable(modVar *tfconfig.Variable) *BlueprintVariable {
 		Required:    modVar.Required,
 		VarType:     modVar.Type,
 	}
-
-	vl, err := structpb.NewValue(modVar.Default)
-	if err == nil {
-		v.DefaultValue = vl
+	if modVar.Default != nil {
+		vl, err := structpb.NewValue(modVar.Default)
+		if err == nil {
+			v.DefaultValue = vl
+		}
 	}
-
 	return v
 }