Ignore empty directories for sources (#530)

Empty directories should not matter for source files, so let's ignore
them for up-to-date checks as well.

This avoids two issues:
 * Right now, the task will be out-of-date when you add an empty
   directory to the source folder. There will also be a build cache miss
   if the only difference between the sources is an empty directory
   somewhere.
 * In Gradle 8.0, the task won't skip any more if there are only empty
   directories in the source folder and no actual files. That is
   because the behaviour for skipping when empty and up-to-date checking
   will be more consistent.

src/main/groovy/com/google/protobuf/gradle/GenerateProtoTask.groovy

@@ -50,6 +50,7 @@ import org.gradle.api.provider.Property
 import org.gradle.api.provider.Provider
 import org.gradle.api.provider.ProviderFactory
 import org.gradle.api.tasks.CacheableTask
+import org.gradle.api.tasks.IgnoreEmptyDirectories
 import org.gradle.api.tasks.Input
 import org.gradle.api.tasks.InputFiles
 import org.gradle.api.tasks.Internal
@@ -302,8 +303,9 @@ public abstract class GenerateProtoTask extends DefaultTask {
   }
 
   @SkipWhenEmpty
-  @InputFiles
   @PathSensitive(PathSensitivity.RELATIVE)
+  @IgnoreEmptyDirectories
+  @InputFiles
   FileCollection getSourceFiles() {
     return sourceFiles
   }