Customize: Move control's fallback selection of default content template to...

Customize: Move control's fallback selection of default content template to `renderContent` method to align with sections and panels.

* Only use default control content template when a more specific template doesn't exist.
* Remove extraneous whitespace from being output in `WP_Customize_Control::render()` method.
* Move Custom Header template printing to `customize_controls_print_footer_scripts` action.

See #30738.

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git-svn-id: https://core.svn.wordpress.org/trunk@41769 1a063a9b-81f0-0310-95a4-ce76da25c4cd