Remove final modifier from com.google.gson.TypeAdapter.nullSafe()

`TypeAdapter.nullSafe()` is now used when a `TypeAdapter` is configured via `@JsonAdapter` and `@JsonAdapter.nullSafe` wasn't set to false.

Existing `TypeAdapter` implementations may already have handled the null-case - the wrapper introduced by calling `TypeAdapter.nullSafe()` is then at best unnecessary or may even break existing code. Clients are therefore forced to review any use of `@JsonAdapter` and set `@JsonAdapter.nullSafe` accordingly - which is an error prone process.

But if `TypeAdapter.nullSafe()` would not be final a `TypeAdapter` implementation which is already nullSafe could simply override the method and return `this`. This especially holds true for the wrapper implementation returned by `TypeAdapter.nullSafe()`.