[MNG-7127] fix MavenCliTest.testStyleColors test in JDK 16

The MavenCliTest.testStyleColors is not failing when the build
is under JDK 8 or JDK 11.

After changing to JDK 16, the test fails, this commit is to fix
the NullPointerException.

Tested on JDK 8, 11 adn 16 with:

`mvn clean verify`
diff --git a/maven-embedder/src/test/java/org/apache/maven/cli/MavenCliTest.java b/maven-embedder/src/test/java/org/apache/maven/cli/MavenCliTest.java
index f9dbf08..cf243fb 100644
--- a/maven-embedder/src/test/java/org/apache/maven/cli/MavenCliTest.java
+++ b/maven-embedder/src/test/java/org/apache/maven/cli/MavenCliTest.java
@@ -322,6 +322,7 @@
 
         MessageUtils.setColorEnabled( true );
         request = new CliRequest( new String[] { "-l", "target/temp/mvn.log" }, null );
+        request.workingDirectory = "target/temp";
         cli.cli( request );
         cli.properties( request );
         cli.logging( request );
@@ -343,6 +344,7 @@
 
         MessageUtils.setColorEnabled( false );
         request = new CliRequest( new String[] { "-Dstyle.color=always", "-B", "-l", "target/temp/mvn.log" }, null );
+        request.workingDirectory = "target/temp";
         cli.cli( request );
         cli.properties( request );
         cli.logging( request );
@@ -350,6 +352,7 @@
 
         MessageUtils.setColorEnabled( false );
         CliRequest maybeColorRequest = new CliRequest( new String[] { "-Dstyle.color=maybe", "-B", "-l", "target/temp/mvn.log" }, null );
+        request.workingDirectory = "target/temp";
         cli.cli( maybeColorRequest );
         cli.properties( maybeColorRequest );
         assertThrows(