Do not set job timeout extra property if None

Author: bmwant

google/cloud/bigquery/job/base.py

@@ -219,7 +219,8 @@ def job_timeout_ms(self, value):
             )
 
         """ Docs indicate a string is expected by the API """
-        self._properties["jobTimeoutMs"] = str(value)
+        if value is not None:
+            self._properties["jobTimeoutMs"] = str(value)
 
     @property
     def labels(self):

tests/unit/job/test_base.py

@@ -1320,3 +1320,9 @@ def test_job_timeout_ms(self):
         # Confirm that integers get converted to strings.
         job_config.job_timeout_ms = 5000
         assert job_config.job_timeout_ms == "5000"  # int is converted to string
+
+    def test_job_timeout_is_none_when_set_none(self):
+        job_config = self._make_one()
+        job_config.job_timeout_ms = None
+        # Confirm value is None and not literal string 'None'
+        assert job_config.job_timeout_ms is None