Remove usage of plexus StringUtils

[mattnelson]

When using this plugin with maven-3.9.0, ClassNotFoundException and NoClassDefFoundError are being thrown because it appears like org.codehaus.plexus.util.StringUtils is no longer provided by maven.

The only usage of that class was checking a string for empty, refactored that usage to do the empty checks directly.

Caused by: java.lang.NoClassDefFoundError: org/codehaus/plexus/util/StringUtils
    at org.gaul.modernizer_maven_plugin.ModernizerMojo.execute (ModernizerMojo.java:193)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2 (MojoExecutor.java:342)

Caused by: java.lang.ClassNotFoundException: org.codehaus.plexus.util.StringUtils
    at org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy.loadClass (SelfFirstStrategy.java:50)
    at org.codehaus.plexus.classworlds.realm.ClassRealm.unsynchronizedLoadClass (ClassRealm.java:271)
    at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass (ClassRealm.java:247)
    at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass (ClassRealm.java:239)
    at org.gaul.modernizer_maven_plugin.ModernizerMojo.execute (ModernizerMojo.java:193)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)

[gaul]

Thank you for your contribution @mattnelson! Did we miss some deprecation warning or other way to know that this would be removed? I will run a new release in a few weeks to pick up this and the Java 20 fix.

[mattnelson]

Another extension appears to have a similar issue > https://www.mail-archive.com/[email protected]/msg128906.html

Because there was a simple refactoring option available, I didn't dig in too much deeper.

see also > gitflow-incremental-builder/gitflow-incremental-builder#637