[MJAVADOC-329] Allow generation of empty javadoc JARs

[jfallows]

Add generateIfEmpty property to javadoc:jar goal, defaulting to false for backwards compatibility.
Add testGenerateIfEmpty property to javadoc:test-jar goal for consistency, defaulting to false for backwards compatibility.

          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-antrun-plugin</artifactId>
            <version>3.0.0</version>
            <executions>
              <execution>
                <phase>package</phase>
                <configuration>
                  <target>
                    <mkdir dir="${project.build.directory}/apidocs" />
                  </target>
                </configuration>
                <goals>
                  <goal>run</goal>
                </goals>
              </execution>
            </executions>
          </plugin>
          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-javadoc-plugin</artifactId>
            <version>3.2.0</version>
            <configuration>
              <sourceFileExcludes>
                <exclude>**/internal/**/*.java</exclude>
              </sourceFileExcludes>
            </configuration>
            <executions>
              <execution>
                <goals>
                  <goal>jar</goal>
                </goals>
              </execution>
            </executions>
          </plugin>

where no non-internal classes exist, therefore no public Javadoc is required, yet Maven Central requires inclusion of a Javadoc JAR during deployment.

This now simplifies to

          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-javadoc-plugin</artifactId>
            <version>3.2.1</version>
            <configuration>
              <sourceFileExcludes>
                <exclude>**/internal/**/*.java</exclude>
              </sourceFileExcludes>
              <generateIfEmpty>true</generateIfEmpty>
            </configuration>
            <executions>
              <execution>
                <goals>
                  <goal>jar</goal>
                </goals>
              </execution>
            </executions>
          </plugin>

Integration tests passed successfully.

[INFO] --- maven-invoker-plugin:3.2.1:verify (integration-test) @ maven-javadoc-plugin ---
[INFO] -------------------------------------------------
[INFO] Build Summary:
[INFO]   Passed: 54, Failed: 0, Errors: 0, Skipped: 17
[INFO] -------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------

As ASF Committer jfallows,

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

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[elharo]

I can review if you resolve conflicts.