BaseTools: Fixed the build fail issue for cases

BaseTools: Fixed the build fail issue for cases

https://bugzilla.tianocore.org/show_bug.cgi?id=1386
This patch is going to fix the regression issue that is
introduced by commit 72a1d77694d51914c0dd6aa97dbfa58634b0a4a5

The issue will happen in the following cases:
1. There is no Pcd value assignment in Dsc file
2. There are duplicate Pcd filed assignment

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: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>