Handle case when pixel values are undefined or null

IPython/html/static/widgets/js/widget.js

@@ -611,7 +611,7 b' define(["widgets/js/manager",'
             /**
              * Makes browser interpret a numerical string as a pixel value.
              */
-            if (/^\d+\.?(\d+)?$/.test(value.trim())) {
+            if (value && /^\d+\.?(\d+)?$/.test(value.trim())) {
                 return value.trim() + 'px';
             }
             return value;