Add a non-service-version test case

Author: basepi

tests/client/client_tests.py

@@ -872,10 +872,11 @@ def test_backdating_transaction(elasticapm_client):
 @pytest.mark.parametrize(
  "elasticapm_client,expected",
  [
- ({"service_version": "v2"}, "v2"),
- ({"service_version": "v2 \x00"}, "v2 _"),
+ ({"service_version": "v2"}, " v2"),
+ ({"service_version": "v2 \x00"}, " v2 _"),
+ ({}, ""),
  ],
  indirect=["elasticapm_client"],
 )
 def test_user_agent(elasticapm_client, expected):
- assert elasticapm_client.get_user_agent() == "apm-agent-python/unknown (myapp {})".format(expected)
+ assert elasticapm_client.get_user_agent() == "apm-agent-python/unknown (myapp{})".format(expected)