Don't allow empty changed-files objects through

__tests__/changedFiles.test.ts

@@ -159,6 +159,15 @@ describe('toChangedFilesMatchConfig', () => {
       });
     });
 
+    describe('but it has empty values', () => {
+      const config = {'changed-files': []};
+
+      it(`returns an empty object`, () => {
+        const result = toChangedFilesMatchConfig(config);
+        expect(result).toEqual<ChangedFilesMatchConfig>({});
+      });
+    });
+
     describe('and there is an unknown value in the config', () => {
       const config = {'changed-files': [{any: 'testing'}, {sneaky: 'test'}]};
 

src/changedFiles.ts

@@ -54,7 +54,10 @@ export function toChangedFilesMatchConfig(
   };
 
   if (Array.isArray(changedFilesConfig)) {
-    if (changedFilesConfig.every(entry => typeof entry === 'string')) {
+    if (
+      changedFilesConfig.length &&
+      changedFilesConfig.every(entry => typeof entry === 'string')
+    ) {
       changedFilesMatchConfig.changedFiles = {
         any: changedFilesConfig
       };
@@ -80,6 +83,11 @@ export function toChangedFilesMatchConfig(
     }
   }
 
+  // If no items were added to `changedFiles` then return an empty object
+  if (!Object.keys(changedFilesMatchConfig.changedFiles).length) {
+    return {};
+  }
+
   return changedFilesMatchConfig;
 }