A check to detect javadoc plugin malfunction

[suztomo]

Let's implement a check to detect javadoc plugin malfunction. We already have test, lint, and clirr jobs in https://github.com/googleapis/java-shared-config/blob/main/.github/workflows/downstream-maven-plugins.yaml.

@alicejli observed the following error:

Execution default-cli of goal org.apache.maven.plugins:maven-javadoc-plugin:3.6.0:aggregate failed: If a non-null excludes list is given, all elements must be non-null -> [Help 1]

[alicejli]

The failure occurs when using the doclet to create the javadocs.

To get a local copy of the doclet, clone https://github.com/googleapis/java-docfx-doclet/tree/main/third_party/docfx-doclet-143274 and checkout 1.9.0 (this is the version of the doclet that Kokoro is using).

Run mvn compile && mvn package and get the path to the generated jar with dependencies. It will look something like this:

/home/alicejli/testRepos/java-docfx-doclet/third_party/docfx-doclet-143274/target/docfx-doclet-1.0-SNAPSHOT-jar-with-dependencies.jar

Then to replicate the javadoc failure, checkout java-bigtable and run the following (be sure to replace the {docletPath} with the path from above):

 mvn clean -B -ntp               -P docFX               -DdocletPath="${docletPath}"               -Dclirr.skip=true               -Denforcer.skip=true               -Dorg.slf4j.simpleLogger.showDateTime=true -Dorg.slf4j.simpleLogger.dateTimeFormat=HH:mm:ss:SSS               -Dcheckstyle.skip=true               -Dflatten.skip=true               -Danimal.sniffer.skip=true               javadoc:aggregate

[alicejli]

Fixed with #662