Fix NPE in HttpServletResponseExtension.getMimeTypes() for parallel test execution

Add null-safe operator when accessing webRequest.applicationContext to handle
cases where GrailsWebRequest.lookup() returns null during parallel test runs.

This fixes the Windows CI failure in RespondMethodSpec where tests that rely on
default HTML content type negotiation were failing with NullPointerException.

Author: jamesfredley

grails-mimetypes/src/main/groovy/org/grails/web/mime/HttpServletResponseExtension.groovy

@@ -81,7 +81,7 @@ class HttpServletResponseExtension {
 
             final webRequest = GrailsWebRequest.lookup()
 
-            def context = webRequest.applicationContext
+            def context = webRequest?.applicationContext
             if (context) {
                 try {
                     mimeTypes = context.getBean(MimeUtility).getKnownMimeTypes() as MimeType[]