config-loader: avoid error on empty config

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/long-wasps-wait.md

@@ -0,0 +1,5 @@
+---
+'@backstage/config-loader': patch
+---
+
+Make schema processing gracefully handle an empty config.

packages/config-loader/src/schema/compile.ts

@@ -190,7 +190,7 @@ export function compileConfigSchemas(
   });
 
   return configs => {
-    const config = ConfigReader.fromConfigs(configs).get();
+    const config = ConfigReader.fromConfigs(configs).getOptional();
 
     visibilityByDataPath.clear();
     deepVisibilityByDataPath.clear();