external/s3: fix destroy() after zero requests (#1684)

Noted while working on getodk/central#1496

Closes getodk/central#1498

Author: alxndrsn

lib/external/s3.js

@@ -29,6 +29,8 @@ const init = (config) => {
   function destroy() {
     destroyed = true;
     return new Promise(resolve => {
+      if (!inflight.size) return resolve(); // eslint-disable-line no-promise-executor-return
+
       let remaining = 0;
       for (const req of inflight) {
         ++remaining; // eslint-disable-line no-plusplus

test/unit/external/s3.js

@@ -59,6 +59,16 @@ describe('external/s3', () => {
     }
   });
 
+  describe('destroy()', () => {
+    it('should resolve when no requests were made', async () => {
+      // given
+      const singleUseS3 = init(s3Config);
+
+      // expect
+      should(await singleUseS3.destroy()).be.undefined();
+    });
+  });
+
   describe('deleteObjsFor()', () => {
     it('should return details for upstream permission error', async () => {
       // given