fix: teach FormBuilder where to look for default values (#2374)

UMap will look into the schema for default values, but untill now
FormBuilder wasn't.
This fix is not the real fix we want, but let's make this last until
then.
The real fix is to refactor schema and make obj.properties a Proxy
object that will then be consumed by FormBuilder as it is now.

umap/static/umap/js/umap.forms.js

@@ -1192,6 +1192,21 @@ U.FormBuilder = L.FormBuilder.extend({
     }
   },
 
+  getter: function (field) {
+    const path = field.split('.')
+    let value = this.obj
+    let sub
+    for (sub of path) {
+      try {
+        value = value[sub]
+      } catch {
+        console.log(field)
+      }
+    }
+    if (value === undefined) values = U.SCHEMA[sub]?.default
+    return value
+  },
+
   finish: (event) => {
     event.helper?.input?.blur()
   },