* Add the Truth dependency
* Port Junit test to Truth in the package `com.google.gson` of the module `gson`
* Replace the `assertThat(e.getMessage()).isEqualTo(...)"` with `assertThat(e).hasMessageThat().isEqualTo(...)`
* Minor fixes
Gson does not actually use the specified LongSerializationPolicy but instead
uses type adapters which emulate the behavior. However, previously Gson's
implementation did not match LongSerializationPolicy regarding null handling.
Because it is rather unlikely that LongSerializationPolicy has been used on
its own, this commit adjusts its implementation to match Gson's behavior
(instead of the other way around).