BaseTools: Allow empty value for HiiPcd in Dsc

BaseTools: Allow empty value for HiiPcd in Dsc

https://bugzilla.tianocore.org/show_bug.cgi?id=1466

DEC file defines PCD default value and PCD supported type.
DSC can configure PCD type and value.
If the value is same to default value in DEC file,
DSC can only configure PCD type and leave empty for value.
This usage supports all type PCD except for DynamicHii type.
So, DynamicHii PCD should support this usage. Below is one example in DSC.

for example,
[PcdsDynamicHii.common.DEFAULT]
PcdPkgTokenSpaceGuid.PcdCName|L"VarName"|gVarGuid|0x00||NV,BS

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>