lisa.target: Fix Target.from_custom_cli() --conf when missing platform-info
lisa.target: Fix Target.from_custom_cli() --conf when missing platform-info

Avoid raising when no platform-info toplevel key is found in the configuration.

ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa