multirelease: Disable maven-jar-plugin for POM artifacts

maven-install-plugin 3.1.2 changed its behavior when trying to run
maven-jar-plugin on <type>pom</type> artifacts. This broke our
multirelease configuration.

Disable maven-jar-plugin for these artifacts (using a profile activated
by the "packaging" property).

Also disable our specific invocation of the maven-jar-plugin if the jar
would be empty, just in case.

https://issues.apache.org/jira/browse/MINSTALL-201

kohlschutter-parent-multirelease/pom.xml

@@ -378,6 +378,7 @@
                                 </goals>
                                 <phase>${kohlschutter.multirelease.jar.phase}</phase>
                                 <configuration>
+                                    <skipIfEmpty>true</skipIfEmpty>
                                     <archive>
                                         <manifestEntries>
                                             <Multi-Release>true</Multi-Release>
@@ -392,6 +393,28 @@
             </build>
         </profile>
 
+        <profile>
+            <id>disable-jar-plugin-for-pom</id>
+            <activation>
+                <property>
+                    <name>packaging</name>
+                    <value>pom</value>
+                </property>
+            </activation>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-jar-plugin</artifactId>
+                        <configuration>
+                            <skipIfEmpty>true</skipIfEmpty>
+                            <classifier>disabled</classifier><!-- just in case -->
+                        </configuration>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
+
         <profile>
             <id>multirelease-java9</id>
             <activation>