Ensure MavenProject#getPluginManagement never returns null

[slawekjaranowski]

When we have pluginManagement in Super POM - MavenProject#getPluginManagement never returns null, should be the same after removing pluginManagement.

It can break the plugin which can assume that it is not null.

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[slawekjaranowski]

example:

• https://github.com/apache/maven-artifact-plugin/blob/f196c7036374093d257358cb7e2e1411c882d185/src/main/java/org/apache/maven/plugins/artifact/buildinfo/PluginUtil.java#L65