SecurityPkg/TPM: measure UEFI images without associated device paths again

SecurityPkg/TPM: measure UEFI images without associated device paths again

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2652

DxeTpm2MeasureBootHandler() and DxeTpmMeasureBootHandler() functions may
receive a FileBuffer argument that is not associated with any particular
device path (e.g., because the UEFI image has not been loaded from any
particular device path).
Therefore rejecting (File==NULL) at the top of the function is invalid.

Fixes: 4b026f0d5af36faf3a3629a3ad49c51b5b3be12f
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Guomin Jiang <guomin.jiang@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>