[BUG] PageId argument not set when adding a new Metadata field

[burningice2866]

Consider this TypeForm for a page metadata where we fill a dropdown based on the page the metadata instance is on

<cms:formdefinition xmlns:cms="http://www.composite.net/ns/management/bindingforms/1.0" xmlns="http://www.composite.net/ns/management/bindingforms/std.ui.controls.lib/1.0" xmlns:ff="http://www.composite.net/ns/management/bindingforms/std.function.lib/1.0" xmlns:f="http://www.composite.net/ns/function/1.0">
  <cms:bindings>
    <cms:binding name="PageId" type="System.Guid" />
    <cms:binding name="BuildingId" type="System.Guid" />
  </cms:bindings>
  <cms:layout>
    <cms:layout.label>
      <cms:read source="PageId" />
    </cms:layout.label>

    <FieldGroup Label="Page building profile">
      <KeySelector Label="BuildingId" Help="" OptionsKeyField="Key" OptionsLabelField="Value" Required="true">
        <KeySelector.Selected>
          <cms:bind source="BuildingId" />
        </KeySelector.Selected>

        <KeySelector.Options>
          <ff:StaticMethodCall Type="Schoolwize.C1Console.ConsoleHelpers, Schoolwize.SchoolWebsite" Method="GetBuildingsForWebsite">
            <ff:StaticMethodCall.Parameters>
              <cms:read source="PageId" />
            </ff:StaticMethodCall.Parameters>
          </ff:StaticMethodCall>
        </KeySelector.Options>
      </KeySelector>
    </FieldGroup>
  </cms:layout>
</cms:formdefinition>

Now, while this works fine when editing pages that has the metadata, the PageId argument is empty when adding the metadata field the first time. And since the value is required, we end up in a chicken-and-egg situation where the field can never be added since it require the value to be set. Which we can't since the dropdown selector is empty because PageId is empty.