avoid NPE in escapeValue()

groovy-ldap/src/main/java/org/xbib/groovy/ldap/LDAP.java

@@ -384,6 +384,9 @@ public class LDAP {
     }
 
     public static String escapeValue(String filter) {
+        if (filter == null) {
+            return null;
+        }
         StringBuilder sb = new StringBuilder();
         for (int i = 0; i < filter.length(); i++) {
             switch (filter.charAt(i)) {
@@ -423,5 +426,4 @@ public class LDAP {
         }
         return sb.toString();
     }
-
 }