fix: don't require a data attribute to be passed when sending a message

src/publisher/index.ts

@@ -154,11 +154,21 @@ export class Publisher {
   publishMessage(message: PubsubMessage, callback: PublishCallback): void {
     const {data, attributes = {}} = message;
 
-    if (!(data instanceof Buffer)) {
+    // We must have at least one of:
+    //   - `data` as a Buffer
+    //   - `attributes` that are not empty
+    if (data && !(data instanceof Buffer)) {
       throw new TypeError('Data must be in the form of a Buffer.');
     }
 
-    for (const key of Object.keys(attributes!)) {
+    const keys = Object.keys(attributes!);
+    if (!data && keys.length === 0) {
+      throw new TypeError(
+        'If data is undefined, at least one attribute must be present.'
+      );
+    }
+
+    for (const key of keys) {
       const value = attributes![key];
       if (typeof value !== 'string') {
         throw new TypeError(`All attributes must be in the form of a string.

test/publisher/index.ts

@@ -243,6 +243,18 @@ describe('Publisher', () => {
       );
     });
 
+    it('should throw an error if data and attributes are both empty', () => {
+      assert.throws(
+        () => publisher.publishMessage({}, spy),
+        /at least one attribute must be present/
+      );
+    });
+
+    it('should allow sending only attributes', () => {
+      const attributes = {foo: 'bar'} as {};
+      assert.doesNotThrow(() => publisher.publishMessage({attributes}, spy));
+    });
+
     it('should throw an error if attributes are wrong format', () => {
       const attributes = {foo: {bar: 'baz'}} as {};