make domain optional in ProteinDomainComplex form

fix #77

ippisite/ippidb/forms.py

@@ -252,9 +252,9 @@ class ComplexCompositionForm(forms.Form):
     )
     domain = forms.ModelChoiceField(
         queryset=Domain.objects.none(),
-        required=True,
+        required=False,
         widget=forms.Select(attrs={'class': 'form-control'}),
-        empty_label=None,
+        empty_label="Unidentified",
     )
     ppc_copy_nb = forms.IntegerField(
         label="Number of copies of the protein in the complex",
@@ -289,7 +289,6 @@ class ComplexCompositionForm(forms.Form):
                 self.cleaned_data.get("ppp_copy_nb_per_p", None) is None:
             self.add_error("ppp_copy_nb_per_p", _("This field is required"))
 
-
 class ComplexCompositionBaseFormSet(forms.BaseFormSet):
     def full_clean(self):
         """