Fix validation to allow blank web tree start URL

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

layerindex/forms.py

@@ -38,6 +38,7 @@ class SubmitLayerForm(forms.ModelForm):
 
     def clean_vcs_web_tree_base_url(self):
         url = self.cleaned_data['vcs_web_tree_base_url'].strip()
+        if url:
             val = URLValidator(verify_exists=False)
             val(url)
         return url