fix(addressbook): Fix broken image in shared address book when image is null

SoObjects/Contacts/SOGoContactLDIFEntry.h

@@ -27,6 +27,7 @@
 @class NSDictionary;
 @class NSString;
 @class NGVList;
+@class NSNull;
 
 @interface SOGoContactLDIFEntry : SOGoObject <SOGoContactObject>
 {

SoObjects/Contacts/SOGoContactLDIFEntry.m

@@ -162,7 +162,7 @@
 
 - (BOOL) hasPhoto
 {
-  return ([ldifEntry objectForKey: @"photo"] != nil);
+  return ([ldifEntry objectForKey: @"photo"] != nil && [ldifEntry objectForKey: @"photo"] != [NSNull null]);
 }
 
 - (NSString *) davEntityTag