Tooltips are only suppressed when a title function returns undefined.

docs/graph.html

@@ -419,8 +419,8 @@ var nodes = [
     <td>string | function</td>
     <td>no</td>
     <td>Title to be displayed when the user hovers over the node.
-      The title can contain HTML code. If using a function, return a falsey
-      value to prevent the tooltip from being displayed.</td>
+      The title can contain HTML code. If using a function, returning <code>undefined</code>
+      will prevent the tooltip from being displayed.</td>
   </tr>
 
   <tr>
@@ -610,8 +610,8 @@ var edges = [
     <td>string | function</td>
     <td>no</td>
     <td>Title to be displayed when the user hovers over the edge.
-      The title can contain HTML code. If using a function, return a falsey
-      value to prevent the tooltip from being displayed.</td>
+      The title can contain HTML code. If using a function, returning <code>undefined</code>
+      will prevent the tooltip from being displayed.</td>
   </tr>
 
   <tr>