[runDialog] Fix adding entries to empty history

Before adding a new entry to the history, we check that it does not
match the previous entry to cut down the number of duplicate entries.
Part of that condition is a check for a history length greater zero
to avoid an illegal negative index, with the side effect that nothing
is ever added while the history is empty.

https://bugzilla.gnome.org/show_bug.cgi?id=621123

js/ui/runDialog.js

@@ -333,7 +333,8 @@ RunDialog.prototype = {
     _run : function(input, inTerminal) {
         let command = input;
 
-        if (this._history.length > 0 && this._history[this._history.length - 1] != input) {
+        if (this._history.length == 0 ||
+            this._history[this._history.length - 1] != input) {
             this._history.push(input);
             this._saveHistory();
         }