Improve compatibility for max long values in backups

ts/test-both/util/timestampLongUtils_test.ts

@@ -7,6 +7,7 @@ import Long from 'long';
 import {
   getSafeLongFromTimestamp,
   getTimestampFromLong,
+  getTimestampOrUndefinedFromLong,
 } from '../../util/timestampLongUtils';
 
 describe('getSafeLongFromTimestamp', () => {
@@ -46,3 +47,27 @@ describe('getTimestampFromLong', () => {
     assert.equal(getTimestampFromLong(null), 0);
   });
 });
+
+describe('getTimestampOrUndefinedFromLong', () => {
+  it('returns undefined when passed 0 Long', () => {
+    assert.equal(
+      getTimestampOrUndefinedFromLong(Long.fromNumber(0)),
+      undefined
+    );
+  });
+
+  it('returns Number.MAX_SAFE_INTEGER when passed Long.MAX_VALUE', () => {
+    assert.equal(
+      getTimestampOrUndefinedFromLong(Long.MAX_VALUE),
+      Number.MAX_SAFE_INTEGER
+    );
+  });
+
+  it('returns a normal number', () => {
+    assert.equal(getTimestampOrUndefinedFromLong(Long.fromNumber(16)), 16);
+  });
+
+  it('returns undefined for null value', () => {
+    assert.equal(getTimestampOrUndefinedFromLong(null), undefined);
+  });
+});