NullPointerException: Cannot invoke "java.lang.CharSequence.toString()" because "target" is null

### Affected version

3.0.0-M3

### Bug description

Whatever the actual problem is, the plugin should log an actionable error message instead of throwing a `NullPointerException`.

```
auncher.Launcher.launch (Launcher.java:201)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:361)
at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:314)
Caused by: java.lang.NullPointerException: Cannot invoke "java.lang.CharSequence.toString()" because "target" is null
at java.lang.String.replace (String.java:3085)
at jdk.internal.reflect.DirectMethodHandleAccessor.invoke (DirectMethodHandleAccessor.java:103)
at java.lang.reflect.Method.invoke (Method.java:580)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke (UberspectImpl.java:571)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke (UberspectImpl.java:554)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute (ASTMethod.java:221)
at org.apache.velocity.runtime.parser.node.ASTReference.execute (ASTReference.java:368)
at org.apache.velocity.runtime.parser.node.ASTReference.value (ASTReference.java:704)
at org.apache.velocity.runtime.parser.node.ASTExpression.value (ASTExpression.java:75)
at org.apache.velocity.runtime.parser.node.ASTSetDirective.render (ASTSetDirective.java:242)
at org.apache.velocity.runtime.parser.node.SimpleNode.render (SimpleNode.java:439)
at org.apache.velocity.Template.merge (Template.java:358)
at org.apache.velocity.Template.merge (Template.java:262)
at org.apache.maven.plugins.changes.announcement.AnnouncementMojo.processTemplate (AnnouncementMojo.java:624)
at org.apache.maven.plugins.changes.announcement.AnnouncementMojo.doGenerate (AnnouncementMojo.java:579)
at org.apache.maven.plugins.changes.announcement.AnnouncementMojo.doGenerate (AnnouncementMojo.java:536)
at org.apache.maven.plugins.changes.announcement.AnnouncementMojo.execute (AnnouncementMojo.java:519)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2 (MojoExecutor.java:328)
at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute (MojoExecutor.java:316)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:212)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:174)
at org.apache.maven.lifecycle.internal.MojoExecutor.access$000 (MojoExecutor.java:75)
at org.apache.maven.lifecycle.internal.MojoExecutor$1.run (MojoExecutor.java:162)
at org.apache.maven.plugin.DefaultMojosExecutionStrategy.execute (DefaultMojosExecutionStrategy.java:39)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:159)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:105)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:73)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:53)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:118)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:261)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:173)
at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:101)
at org.apache.maven.cli.MavenCli.execute (MavenCli.java:906)
at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:283)
at org.apache.maven.cli.MavenCli.main (MavenCli.java:206)
at jdk.internal.reflect.DirectMethodHandleAccessor.invoke (DirectMethodHandleAccessor.java:103)
at java.lang.reflect.Method.invoke (Method.java:580)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:255)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:201)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:361)
at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:314)
```
Reproduce with:
```
git clone https://github.com/apache/commons-fileupload.git
cd commons-fileupload
git checkout f1028401e3d59bd42aee9ab4d26995991db1aadc
mvn -e --non-recursive changes:announcement-generate -Prelease-notes -Dchanges.version=1.6.0
```

Run the provided Maven reproduction against commit f1028401e3d59bd42aee9ab4d26995991db1aadc, then inspect AnnouncementMojo.processTemplate and doGenerate around the reported AnnouncementMojo.java lines 536, 579, and 624. Done means the same invalid input produces an actionable logged error instead of the NullPointerException.

Written by the indexing model from the issue text.