"Platform" button unresponsive with no targets

[XAMPPRocky]

The platform button in the navbar currently gives no feedback if there are no platforms. I feel it would more appropriate to display something or hide the button entirely.

• Example crate: https://docs.rs/crate/timeago-cli/0.1.3

[jyn514]

Mentoring instructions: In

docs.rs/templates/header/package_navigation.html

Lines 43 to 44 in 5f4f6cf

	{# If there are platforms, show a dropdown with them #}
	{%- if platforms -%}

check whether platforms is empty, not whether it is truthy. https://tera.netlify.app/docs/#control-structures