handle null parameters more gracefully

Author: vladak

plugins/src/main/java/opengrok/auth/plugin/UserWhiteListPlugin.java

@@ -57,6 +57,10 @@ public class UserWhiteListPlugin implements IAuthorizationPlugin {
 
     @Override
     public void load(Map<String, Object> parameters) {
+        if (parameters == null) {
+            throw new IllegalArgumentException("Null parameter");
+        }
+
         String filePath;
         if ((filePath = (String) parameters.get(FILE_PARAM)) == null) {
             throw new IllegalArgumentException("Missing parameter [" + FILE_PARAM + "] in the configuration");

plugins/src/test/java/opengrok/auth/plugin/UserWhiteListPluginTest.java

@@ -117,7 +117,7 @@ public void init(String param) {
     @MethodSource("parameters")
     public void shouldThrowOnLoadIfNullArgument(String param) {
         init(param);
-        assertThrows(NullPointerException.class, () -> {
+        assertThrows(IllegalArgumentException.class, () -> {
             //noinspection ConstantConditions
             plugin.load(null);
             }, "plugin.load(null)");