skip fetching project for empty diffs (#4000)

Signed-off-by: Modular Magician <[email protected]>

google/resource_compute_instance_template.go

@@ -483,13 +483,7 @@ func resourceComputeInstanceTemplateSourceImageCustomizeDiff(diff *schema.Resour
 	for i := 0; i < numDisks; i++ {
 		key := fmt.Sprintf("disk.%d.source_image", i)
 		if diff.HasChange(key) {
-			// project must be retrieved once we know there is a diff to resolve, otherwise it will
-			// attempt to retrieve project during `plan` before all calculated fields are ready
-			// see https://github.com/terraform-providers/terraform-provider-google/issues/2878
-			project, err := getProjectFromDiff(diff, config)
-			if err != nil {
-				return err
-			}
+			var err error
 			old, new := diff.GetChange(key)
 			if old == "" || new == "" {
 				// no sense in resolving empty strings
@@ -499,6 +493,13 @@ func resourceComputeInstanceTemplateSourceImageCustomizeDiff(diff *schema.Resour
 				}
 				continue
 			}
+			// project must be retrieved once we know there is a diff to resolve, otherwise it will
+			// attempt to retrieve project during `plan` before all calculated fields are ready
+			// see https://github.com/terraform-providers/terraform-provider-google/issues/2878
+			project, err := getProjectFromDiff(diff, config)
+			if err != nil {
+				return err
+			}
 			oldResolved, err := resolveImage(config, project, old.(string))
 			if err != nil {
 				return err