BaseTools: Fix one crash bug in the report for Fixed structure Pcd

BaseTools: Fix one crash bug in the report for Fixed structure Pcd

The case is:
in the DSC file:
SKUID_IDENTIFIER               = ALL

[SkuIds]
  0|DEFAULT
  1|A

[PcdsFixedAtBuild.common.A]
  TokenSpaceGuid.Test401|{0x0F, 0x12}
  TokenSpaceGuid.Test401.TEST401INT8ARRAY[0]|'B'

in the build report, Data = OverrideValues[Keys[0]], but the Keys[0]
is the keyword "DEFAULT", and in this case the "DEFAULT" SKU doesn't
save any value, then it cause the Data is empty, in the next code
when we use the code it cause crash.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>