Matchers.isSubtypeOf always matches literal null

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### Description of the problem / feature request:

`Matchers.isSubtypeOf` always matches literal null. This would be correct if we were validating that the value can be cast to the requested type, however literal null should not match isSubtypeOf in most cases.
It's arguable that it could be matched for types `Void` and `Object`.

### Feature requests: what underlying problem are you trying to solve with this feature?

Either return false for null in most cases, or provide javadoc describing the issue.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

isSubtypeOf check on any null literal.

### What version of Error Prone are you using?

2.3.3

### Have you found anything relevant by searching the web?

No, I don't believe this has been reported yet.