fix incorrect `empty-docstring`

in notebook tooltip.

The else condition wasn't quite right, and would set doctsring to 'empty doctsring'
even when there was a doctoring.

IPython/html/static/notebook/js/tooltip.js

@@ -334,11 +334,14 b' var IPython = (function (IPython) {'
         if (docstring == null) {
             docstring = reply.docstring;
         }
-
-        if (docstring == null && this._hide_if_no_docstring) {
-            return;
-        } else {
-            docstring = "<empty docstring>";
+        
+        if (docstring == null) {
+            // For reals this time, no docstring
+            if (this._hide_if_no_docstring) {
+                return;
+            } else {
+                docstring = "<empty docstring>";
+            }
         }
 
         this.tooltip.fadeIn('fast');