Fix '[object object]' label in dropdown

Appears when rendering for the first time OCI's 'Migration Image Map'
field in the Wizard Options page.
This error was related to the fact that the dropdown had a separator.

src/components/molecules/PropertiesTable/index.jsx

@@ -124,7 +124,7 @@ class PropertiesTable extends React.Component<Props> {
       ...items,
     ]
 
-    let selectedItem = items.find(i => i.value === this.props.valueCallback(prop))
+    let selectedItem = items.find(i => !i.separator && i.value === this.props.valueCallback(prop))
 
     return (
       <Dropdown