Various Google error-prone findings

[bpcreech]

Running the most recent Google error-prone over this codebase turns up a few categories of issues, e.g.:

1. (New) Lack of notation of CanIgnoreReturnValue on methods which return this (typically Builder methods). This partially inhibits effective use of CheckReturnValue by users of this library.
2. A few methods that aren't marked @Nullable but clearly return null. This partially inhitibs effective use of tools like NullAway.
3. getClass called on .class, which is redundant
4. Attempts to check for proto lists using == null. Proto fields are never null, these should check .isEmpty().
5. A handful of tests that aren't running because of no @Test annotation (one initially commented out, but that one should use @Ignore... and another test which is not enabled and also doesn't seem to test anything [testFromCurrentContextWithNegativeLevel])
6. Backwards assertEqual(x, y) statements (which yields backwards error messages on failure)
7. One unnecessary @SupressWarnings("unchecked")
8. A few more tiny things

Will send a PR