treat all falsy values the same in tooltip

undefined, null, and empty string are all treated the same.

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

@@ -341,22 +341,22 b' var IPython = (function (IPython) {'
 
         // build docstring
         var defstring = content.call_def;
-        if (defstring == null) {
+        if (!defstring) {
             defstring = content.init_definition;
         }
-        if (defstring == null) {
+        if (!defstring) {
             defstring = content.definition;
         }
 
         var docstring = content.call_docstring;
-        if (docstring == null) {
+        if (!docstring) {
             docstring = content.init_docstring;
         }
-        if (docstring == null) {
+        if (!docstring) {
             docstring = content.docstring;
         }
 
-        if (docstring == null) {
+        if (!docstring) {
             // For reals this time, no docstring
             if (this._hide_if_no_docstring) {
                 return;