allow null prefixes in project factory when override is not set (#3691)

modules/project-factory/README.md

@@ -876,6 +876,7 @@ module "project-factory" {
     locations = {
       storage = "eu"
     }
+    prefix = "foo"
   }
   data_merges = {
     labels = {
@@ -885,9 +886,6 @@ module "project-factory" {
       "compute.googleapis.com"
     ]
   }
-  data_overrides = {
-    prefix = "foo"
-  }
   factories_config = {
     projects = "data/projects"
   }
@@ -897,6 +895,7 @@ module "project-factory" {
 
 ```yaml
 parent: folders/1234567890
+# prefix from defaults (foo)
 services:
   - iam.googleapis.com
   - contactcenteraiplatform.googleapis.com
@@ -907,6 +906,8 @@ services:
 ```yaml
 parent: folders/1234567890
 descriptive_name: "Test Project 1"
+# null prefix
+prefix: null
 services:
   - iam.googleapis.com
   - contactcenteraiplatform.googleapis.com
@@ -915,6 +916,8 @@ services:
 
 ```yaml
 parent: folders/1234567890
+# explicit prefix
+prefix: bar
 services:
   - iam.googleapis.com
   - storage.googleapis.com

modules/project-factory/projects-defaults.tf

@@ -80,13 +80,16 @@ locals {
           local.data_defaults.defaults.parent
         ), null
       )
-      prefix = try( # type: string, nullable
-        coalesce(
-          local.data_defaults.overrides.prefix,
-          try(v.prefix, null),
-          local.data_defaults.defaults.prefix
-        ), null
-      )
+      prefix = try(
+        (
+          local.data_defaults.overrides.prefix != null
+          ? local.data_defaults.overrides.prefix
+          : (
+            try(v.prefix, "-") == "-"
+            ? local.data_defaults.defaults.prefix
+            : v.prefix
+          )
+      ), null)
       project_reuse = ( # type: object({...})
         try(v.project_reuse, null) != null
         ? merge(