[MockModem] Ignore the test when no APN setting in db

Symptom: No suitable APN setting for setup_data_call on cuttlefish.
Root casue: The paris was disabled on Cuttlefish.
Solution: To avoid this issue, we will ignore the test if there is no
APN setting.

Flag: TEST_ONLY
Bug: 347754514
Test: Local test passed.
Change-Id: I38e44cb7017396c9db36e16849f152bfed5d0b84