fix(label): ensure the translation kept is not null

SoObjects/SOGo/NSObject+Utilities.h

@@ -27,6 +27,7 @@
 @class NSArray;
 @class NSString;
 @class WOContext;
+@class NSNull;
 
 @interface NSObject (SOGoObjectUtilities)
 

SoObjects/SOGo/NSObject+Utilities.m

@@ -110,7 +110,7 @@ static NSMutableDictionary *translationCache = nil;
 
   while (!label && (language = [languages nextObject]))
     {
-      if (![translationCache objectForKey: language]) {
+      if (![translationCache objectForKey: language] || [[translationCache objectForKey: language] objectForKey: key] != [NSNull null]) {
         paths = [bundle pathsForResourcesOfType: @"strings"
           inDirectory: [NSString stringWithFormat: @"%@.lproj",
                         language]