fix: if no keys, set number of translated to 0

If there are no string keys for a given key type (sim-specific, shared,
or common), we set the number of translated strings in the form to 0.

For #345.

Came out of phetsims/qa#869.

src/client/js/computeTranslatedStringsData.js

@@ -46,6 +46,12 @@ const computeTranslatedStringsData = translationValues => {
         }
         translatedStringsData[ keyType ].translated = translatedStringsCount;
       }
+      else {
+
+        // If there aren't any string keys for a given key type, set translated to 0.
+        // Thus, you'd get a message like "0 of 0 {key type} strings translated".
+        translatedStringsData[ keyType ].translated = 0;
+      }
     }
   }
   return translatedStringsData;