MdeModulePkg/UefiBootManagerLib: Remove assertion

EfiBootManagerStartHotkeyService() asserts when "BootOptionSupport"
variable doesn't exist.

In fact, though "BootOptionSupport" variable is set in BdsDxe
module, it's possible that the variable is deleted by
PlatformBootManagerBeforeConsole().

The patch removes the assertion and adds code to handle the case.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>