[UNDERTOW-1906] Guard against null resource path

core/src/main/java/io/undertow/server/handlers/resource/CachingResourceManager.java

@@ -77,6 +77,9 @@ public void handleChanges(Collection<ResourceChangeEvent> changes) {
 
     @Override
     public CachedResource getResource(final String p) throws IOException {
+        if( p == null ) {
+            return null;
+        }
         final String path;
         //base always ends with a /
         if (p.startsWith("/")) {

core/src/main/java/io/undertow/server/handlers/resource/ClassPathResourceManager.java

@@ -58,6 +58,9 @@ public ClassPathResourceManager(final ClassLoader classLoader) {
 
     @Override
     public Resource getResource(final String path) throws IOException {
+        if( path == null ) {
+            return null;
+        }
         String modPath = path;
         if(modPath.startsWith("/")) {
             modPath = path.substring(1);

core/src/main/java/io/undertow/server/handlers/resource/PathResourceManager.java

@@ -186,6 +186,9 @@ public PathResourceManager setBase(final File base) {
     }
 
     public Resource getResource(final String p) {
+        if( p == null ) {
+            return null;
+        }
         String path;
         //base always ends with a /
         if (p.startsWith("/")) {

servlet/src/main/java/io/undertow/servlet/spec/ServletContextImpl.java

@@ -296,7 +296,7 @@ public Set<String> getResourcePaths(final String path) {
 
     @Override
     public URL getResource(final String path) throws MalformedURLException {
-        if (!path.startsWith("/")) {
+        if (path == null || !path.startsWith("/")) {
             throw UndertowServletMessages.MESSAGES.pathMustStartWithSlash(path);
         }
         Resource resource = null;