MdeModulePkg Variable: return error for empty str VariableName to GetVariable

MdeModulePkg Variable: return error for empty str VariableName to GetVariable

Current GetVariable implementation will return the first variable for
empty str VariableName, it is because GetVariable and GetNextVariablename
are sharing same function FindVariable.
But UEFI sepc defines SetVariable that If VariableName is an empty string,
then EFI_INVALID_PARAMETER is returned, that means an empty string variable
could never be set successfully, so GetVariable should return error for
empty string VariableName.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>