fixed a crash (GoogleCloudPlatform#5133)

mmv1/products/compute/terraform.yaml

@@ -2274,7 +2274,7 @@ overrides: !ruby/object:Overrides::ResourceOverrides
           If it is not provided, the provider region is used.
       enable: !ruby/object:Overrides::Terraform::PropertyOverride
         custom_expand: 'templates/terraform/custom_expand/bool_to_upper_string.erb'
-        custom_flatten: 'templates/terraform/custom_flatten/string_to_bool.erb'
+        custom_flatten: 'templates/terraform/custom_flatten/string_to_bool_default_true.erb'
         send_empty_value: true
       ipAddress: !ruby/object:Overrides::Terraform::PropertyOverride
         default_from_api: true

mmv1/templates/terraform/custom_flatten/string_to_bool.erb

@@ -1,8 +1,11 @@
 func flatten<%= prefix -%><%= titlelize_property(property) -%>(v interface{}, d *schema.ResourceData, config *Config) interface{} {
-     b, err := strconv.ParseBool(v.(string))
-     if err != nil {
-         // If we can't convert it into a bool return value as is and let caller handle it
-         return v
-     }
-     return b
- }
+    if v == nil {
+        return v
+    }
+    b, err := strconv.ParseBool(v.(string))
+    if err != nil {
+        // If we can't convert it into a bool return value as is and let caller handle it
+        return v
+    }
+    return b
+}

mmv1/templates/terraform/custom_flatten/string_to_bool_default_true.erb

@@ -0,0 +1,11 @@
+func flatten<%= prefix -%><%= titlelize_property(property) -%>(v interface{}, d *schema.ResourceData, config *Config) interface{} {
+    if v == nil {
+        return true
+    }
+    b, err := strconv.ParseBool(v.(string))
+    if err != nil {
+        // If we can't convert it into a bool return value as is and let caller handle it
+        return v
+    }
+    return b
+}