Fix color display for cables

netbox/templates/dcim/cable.html

@@ -56,7 +56,11 @@
                     <tr>
                         <td>Color</td>
                         <td>
-                            <span class="label color-block" style="background-color: #{{ cable.color }}">&nbsp;</span>
+                            {% if cable.color %}
+                                <span class="label color-block" style="background-color: #{{ cable.color }}">&nbsp;</span>
+                            {% else %}
+                                <span class="text-muted">&mdash;</span>
+                            {% endif %}
                         </td>
                     </tr>
                     <tr>