ShellPkg/CommandLib: Use first found UC for unsupported PlatformLang

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2300

On some firmwares PlatformLang is set to the local language (e.g. ru-RU),
however there is no Unicode Collation protocol instance that supports it.
As for missing PlatformLang, fall back to the first found instance.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Marvin Haeuser <mhaeuser@outlook.de>