Support Angular's null translations in JSON

Files generated by ngx-translate can contain null for the translation,
meaning "use default language".

Poedit doesn't support editing this meaning, but it should be able to
read such files and fill in actual translation.

src/catalog_json.cpp

@@ -197,9 +197,17 @@ class GenericJSONItem : public JSONCatalogItem
     GenericJSONItem(int id, const std::string& key, json_t& node) : JSONCatalogItem(id, node)
     {
         m_string = str::to_wx(key);
-        auto trans = str::to_wx(node.get<std::string>());
-        m_translations.push_back(trans);
-        m_isTranslated = !trans.empty();
+        if (node.is_null())
+        {
+            m_translations.push_back(wxString());
+            m_isTranslated = false;
+        }
+        else
+        {
+            auto trans = str::to_wx(node.get<std::string>());
+            m_translations.push_back(trans);
+            m_isTranslated = !trans.empty();
+        }
     }
 
     void UpdateInternalRepresentation() override
@@ -235,9 +243,9 @@ class GenericJSONCatalog : public JSONCatalog
         for (auto& el : node.items())
         {
             auto& val = el.value();
-            if (val.is_string())
+            if (val.is_string() || val.is_null())
             {
-                m_items.push_back(std::make_shared<GenericJSONItem>(++id, prefix + el.key(), el.value()));
+                m_items.push_back(std::make_shared<GenericJSONItem>(++id, prefix + el.key(), val));
             }
             else if (val.is_object())
             {
@@ -337,7 +345,7 @@ class FlutterCatalog : public JSONCatalog
             {
                 auto mi = metadata.find(key);
                 auto meta = (mi != metadata.end()) ? mi->second : nullptr;
-                m_items.push_back(std::make_shared<FlutterItem>(++id, prefix + el.key(), el.value(), meta));
+                m_items.push_back(std::make_shared<FlutterItem>(++id, prefix + el.key(), val, meta));
             }
             else if (val.is_object())
             {

tests/json/de.json

@@ -1,6 +1,8 @@
 {
 	"demo.greeting": "Hallo {{name}}!",
-	"demo.text": "Dies ist eine einfache Applikation um die Funktionen von ngx-translate zu demonstrieren.",
+	"demo.text": "Dies ist eine einfache Applikation um die Funktionen von ngx-translate zu demonstrieren.",
 	"demo.text-in-code": "Verwendung von Übersetzungen im Sourcecode",
-	"demo.title": "Übersetzungs-Demo"
+	"demo.title": "Übersetzungs-Demo",
+	"empty_string_translation": "",
+	"null_translation": null
 }

tests/json/en_dos.json

@@ -2,5 +2,7 @@
 	"demo.greeting": "Hello {{name}}!",
 	"demo.text": "This is a simple demonstration app for ngx-translate",
 	"demo.text-in-code": "Translation used from code",
-	"demo.title": "Translation demo"
+	"demo.title": "Translation demo",
+	"empty_string_translation": "",
+	"null_translation": null
 }