MdeModulePkg/Library/DxeHttpLib: Handle the blank value in HTTP header.

MdeModulePkg/Library/DxeHttpLib: Handle the blank value in HTTP header.

This patch is to resolve the lock-up issue if the value of HTTP header
is blank.  The issue is recorded @
https://bugzilla.tianocore.org/show_bug.cgi?id=1102.

Cc: Stephen Benjamin <stephen@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Tested-by: Stephen Benjamin <stephen@redhat.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>