Fix NPE caused by KotlinCompileMojoBase if no "Xcoroutines" value is passed

NPE was happening in CLICompiler.chooseCoroutinesApplicabilityLevel

libraries/tools/kotlin-maven-plugin/src/main/java/org/jetbrains/kotlin/maven/KotlinCompileMojoBase.java

@@ -452,9 +452,12 @@ public abstract class KotlinCompileMojoBase<A extends CommonCompilerArguments> e
         arguments.suppressWarnings = nowarn;
         arguments.languageVersion = languageVersion;
         arguments.apiVersion = apiVersion;
-        arguments.coroutinesState = experimentalCoroutines;
         arguments.multiPlatform = multiPlatform;
 
+        if (experimentalCoroutines != null) {
+            arguments.coroutinesState = experimentalCoroutines;
+        }
+
         configureSpecificCompilerArguments(arguments);
 
         try {