Fix get_time util if input is 0.

This also fixes `Get Time` keyword using that util. Fixes #4438.

src/robot/utils/robottime.py

@@ -214,7 +214,7 @@ def get_time(format='timestamp', time_=None):
     - Otherwise (and by default) the time is returned as a timestamp string in
       format '2006-02-24 15:08:31'
     """
-    time_ = int(time_ or time.time())
+    time_ = int(time.time() if time_ is None else time_)
     format = format.lower()
     # 1) Return time in seconds since epoc
     if 'epoch' in format:

utest/utils/test_robottime.py

@@ -328,7 +328,10 @@ def test_parse_time_with_now_and_utc(self):
             parsed = parse_time(input.upper().replace('NOW', 'UtC'))
             zone = time.altzone if time.localtime().tm_isdst else time.timezone
             expected += zone
-            assert_true(expected <= parsed <= expected + 1),
+            assert_true(expected <= parsed <= expected + 1)
+
+    def test_get_time_with_zero(self):
+        assert_equal(get_time('epoch', 0), 0)
 
     def test_parse_modified_time_with_invalid_times(self):
         for value, msg in [("-100", "Epoch time must be positive (got -100)."),