Fix null pointer crash with bazel coverage on only incompatible tests

[philsc]

This is a follow-up to 2f1ff6f. That
patch accidentally introduced a crash when running coverage on tests
where all tests are incompatible.

FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.NullPointerException: Null reportGenerator
        at com.google.devtools.build.lib.bazel.coverage.AutoValue_CoverageArgs.<init>(AutoValue_CoverageArgs.java:68)
        at com.google.devtools.build.lib.bazel.coverage.CoverageArgs.create(CoverageArgs.java:58)
        at com.google.devtools.build.lib.bazel.coverage.CoverageReportActionBuilder.createCoverageActionsWrapper(CoverageReportActionBuilder.java:226)
        at com.google.devtools.build.lib.bazel.coverage.BazelCoverageReportModule$1.createCoverageReportActionsWrapper(BazelCoverageReportModule.java:98)
        at com.google.devtools.build.lib.analysis.BuildView.createResult(BuildView.java:558)
        at com.google.devtools.build.lib.analysis.BuildView.update(BuildView.java:492)
        at com.google.devtools.build.lib.buildtool.AnalysisPhaseRunner.runAnalysisPhase(AnalysisPhaseRunner.java:227)
        at com.google.devtools.build.lib.buildtool.AnalysisPhaseRunner.execute(AnalysisPhaseRunner.java:137)
        at com.google.devtools.build.lib.buildtool.BuildTool.buildTargets(BuildTool.java:266)
        at com.google.devtools.build.lib.buildtool.BuildTool.processRequest(BuildTool.java:506)
        at com.google.devtools.build.lib.buildtool.BuildTool.processRequest(BuildTool.java:474)
        at com.google.devtools.build.lib.runtime.commands.TestCommand.doTest(TestCommand.java:148)
        at com.google.devtools.build.lib.runtime.commands.TestCommand.exec(TestCommand.java:113)
        at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.execExclusively(BlazeCommandDispatcher.java:584)
        at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.exec(BlazeCommandDispatcher.java:231)
        at com.google.devtools.build.lib.server.GrpcServerImpl.executeCommand(GrpcServerImpl.java:550)
        at com.google.devtools.build.lib.server.GrpcServerImpl.lambda$run$1(GrpcServerImpl.java:614)
        at io.grpc.Context$1.run(Context.java:566)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.base/java.lang.Thread.run(Unknown Source)

This patch fixes the crash by handling the situation properly. This
results in all tests being skipped and reported as being skipped.

A new test validates the correct behaviour.

(cherry picked from commit 3dd2b93)