Obey method contract

Author: michael-o

src/main/java/org/apache/maven/plugins/javadoc/JavadocUtil.java

@@ -780,12 +780,16 @@ protected static void invokeMaven(
         InvocationRequest request = new DefaultInvocationRequest();
         request.setBaseDirectory(projectFile.getParentFile());
         request.setPomFile(projectFile);
-        request.setGlobalSettingsFile(globalSettingsFile);
-        if (userSettingsFile.isFile()) {
+        if (globalSettingsFile != null && globalSettingsFile.isFile()) {
+            request.setGlobalSettingsFile(globalSettingsFile);
+        }
+        if (userSettingsFile != null && userSettingsFile.isFile()) {
             request.setUserSettingsFile(userSettingsFile);
         }
-        request.setGlobalToolchainsFile(globalToolchainsFile);
-        if (userToolchainsFile.isFile()) {
+        if (globalToolchainsFile != null && globalToolchainsFile.isFile()) {
+            request.setGlobalToolchainsFile(globalToolchainsFile);
+        }
+        if (userToolchainsFile != null && userToolchainsFile.isFile()) {
             request.setToolchainsFile(userToolchainsFile);
         }
         request.setBatchMode(true);