fix: skip plugins without processor_classes in annotation processor classification

[jianingxu1]

Problem

Non-annotation-processor plugins are being misclassified as annotation processors.

Context:
We are using kt_jvm_library with mixed Java/Kotlin sources, and we want to use Error Prone as a java_plugin via the plugins attribute, so that Error Prone runs as a javac plugin on the Java sources.

Problem:
When no annotation processors are passed in, and Error Prone is passed, running bazel build <target> fails on javac step, throwing
error: package <package> does not exist and error: cannot find symbol errors on Kotlin pkgs/symbols.

An Error Prone java_plugin has processor_jars but no processor_classes. However, _targets_to_annotation_processors includes it because it only checks for processor_jars, misclassifying it as an annotation processor.

This causes:

1. KAPT is triggered but the src_main-kapt-generated-stub.jar in kt_stubs_for_java is empty (since there were no real processors to generate stubs). Code reference.
2. The Kotlin compile jar is not added to kt_stubs_for_java (compile.bzl:968). It is only populated when annotation_processors is empty (given Kotlin sources are present).
3. When building Java, Java sources that reference Kotlin symbols fail with missing symbols errors.

In Bazel, java_plugin covers both annotation processors (declare processor_class) and javac plugins like Error Prone (no processor_class). Only the former should go through KAPT.

Note that: if both ErrorProne and an annotator processor are provided, this issue does not happen.

Fix

In _targets_to_annotation_processors (kotlin/internal/jvm/plugins.bzl), also require processor_classes to be non-empty:

- if p.processor_jars:
+ if p.processor_jars and p.processor_classes.to_list():

Error Prone is still picked up correctly as a javac plugin via the plugins attribute in _run_kt_builder_action, it just no longer incorrectly triggers KAPT and not passing kt_stubs_for_java.

Testing

• bazel test //src/...

[restingbull]

Please add a repro case for this -- we'd rather not have it break later silently later on.