fix: only set per file $extends environment override if it exists

Author: johnb8

app-config-extensions/src/extends-directive.ts

@@ -48,9 +48,12 @@ function fileReferenceDirective(keyName: string, meta: ParsedValueMetadata): Par
 
           const environmentOptions = {
             ...environmentOptionsFromContext(context),
-            override: env,
           };
 
+          if (env) {
+            environmentOptions.override = env;
+          }
+
           const parsed = await resolvedSource
             .read(extensions, {
               ...context,