readline: handle null completer graciously

Fixes #1698.

lib/readline.js

@@ -45,6 +45,12 @@ function Interface(input, output, completer) {
   }
   EventEmitter.call(this);
 
+  completer = completer || function() { return []; };
+
+  if (typeof completer !== 'function') {
+    throw new TypeError("Argument 'completer' must be a function");
+  }
+
   var self = this;
 
   this.output = output;
@@ -605,9 +611,7 @@ Interface.prototype._ttyWrite = function(s, key) {
         break;
 
       case 'tab': // tab completion
-        if (this.completer) {
-          this._tabComplete();
-        }
+        this._tabComplete();
         break;
 
       case 'left':