Bug 1597540 - Provide default spellchecker.dictionary for suite. r=IanN

l10n builds might not provide this in suite-l10n.js and composing messages will
fail then.

suite/app/profile/suite-prefs.js

@@ -366,6 +366,7 @@ pref("mailnews.ui.deleteMarksRead", true);
 // (This is intentionally on the high side; see bugs 746055 and 768015.)
 pref("image.mem.max_decoded_image_kb", 256000);
 
+pref("spellchecker.dictionary", "");
 pref("spellchecker.dictionaries.download.url", "https://addons.thunderbird.net/%LOCALE%/%APP%/dictionaries");
 
 // this will automatically enable inline spellchecking (if it is available) for

suite/components/nsSuiteGlue.js

@@ -1147,13 +1147,9 @@ SuiteGlue.prototype = {
       }
     } catch (ex) {}
 
-    // try to get dictionary preference and initialize with blank if not set
+    // Try to get dictionary preference and adjust if not valid.
     var prefName = "spellchecker.dictionary";
-    var prefValue = "";
-
-    try {
-      prefValue = Services.prefs.getCharPref(prefName);
-    } catch (ex) {}
+    var prefValue = Services.prefs.getCharPref(prefName);
 
     // replace underscore with dash if found in language
     if (/_/.test(prefValue)) {