Support running all available patch checks

[copybara-service]

Support running all available patch checks

The BaseErrorProneJavaCompiler supports running all patch checks; it just requires that one doesn't provide any explicitly named checkers.

In practice this doesn't work, however, because:

• PatchingOptions requires a named checker or a custom refactorer.
• Specifying -XepPatchChecks: without any explicit check will cause the empty string to be stored as a named checker.

This PR proposes to resolve the above issues by:

1. Requiring -XepPatchLocation when -XepPatchChecks is specified, but not vice versa.
2. Omitting empty strings from -XepPatchChecks:.

Fixes #947

FUTURE_COPYBARA_INTEGRATE_REVIEW=#947 from PicnicSupermarket:bugfix/support-empty-PatchChecks c5d9125