Remove object_type default

[sbussetti]

Fixes #700.

Discussion: https://wordpress.org/support/topic/term-meta-is-saved-but-it-doesnt-appear-in-term-edit-screen-later/page/2/#post-8259773

So order matters here due to the caching pattern used in CMB2_

        public function object_type( $object_type = '' ) {
                if ( $object_type ) {
                        $this->object_type = $object_type;
                        return $this->object_type;
                }

                if ( $this->object_type ) {
                        return $this->object_type;
                }

                $this->object_type = $this->current_object_type();

                return $this->object_type;
        }

Which I think explains why different folks report different results. Depending on the order things get initialized in, object_type could be set differently.

However, in the case where nothing else is setting object_type (this only happens during render_form_open and render_form_close), then in the above logic, current_object_type is NEVER called, because $this->object_type is already defaulted to 'post'.

If you remove that default, everything works great!

Changes proposed in this pull request

• Sets default object_type to null for CMB2_Base

[jtsternberg]

Many have reported this as fixing issues with term meta not saving, so merging. Thank you for submitting.