address comments

packages/config-loader/src/lib/reader.ts

@@ -14,11 +14,11 @@
  * limitations under the License.
  */
 
-import yaml from 'yaml';
+import { AppConfig, JsonObject, JsonValue } from '@backstage/config';
 import { basename } from 'path';
-import { isObject } from './utils';
-import { JsonValue, JsonObject, AppConfig } from '@backstage/config';
+import yaml from 'yaml';
 import { ReaderContext } from './types';
+import { isObject } from './utils';
 
 /**
  * Reads and parses, and validates, and transforms a single config file.
@@ -67,9 +67,12 @@ export async function readConfigFile(
     const out: JsonObject = {};
 
     for (const [key, value] of Object.entries(obj)) {
-      const result = await transform(value, `${path}.${key}`);
-      if (result !== undefined) {
-        out[key] = result;
+      // undefined covers optional fields
+      if (value !== undefined) {
+        const result = await transform(value, `${path}.${key}`);
+        if (result !== undefined) {
+          out[key] = result;
+        }
       }
     }
 

packages/config-loader/src/lib/secrets.ts

@@ -122,7 +122,7 @@ export async function readSecret(
     const { path } = secret;
     const parts = typeof path === 'string' ? path.split('.') : path;
 
-    let value: JsonValue = await parser(content);
+    let value: JsonValue | undefined = await parser(content);
     for (const [index, part] of parts.entries()) {
       if (!isObject(value)) {
         const errPath = parts.slice(0, index).join('.');
@@ -132,6 +132,7 @@ export async function readSecret(
       }
       value = value[part];
     }
+
     return String(value);
   }