BaseTools/BuildReport: fix report for platforms/arches without struct PCDs

BaseTools/BuildReport: fix report for platforms/arches without struct PCDs

The goal of commit 97c8f5b9e7d3 ("BaseTools:StructurePCD value display
incorrect in "Not used" section.", 2019-02-02) was to display the full
contents of such structure PCDs in the build report that were set in the
platform DSC or the FDF, but not used in any module INFs. The listings
would appear in the

  PCDs not used by modules or in conditional directives

section of the build report.

Commit 97c8f5b9e7d3 assumed that any (platform, architecture) combination
would have a (possibly empty) set of structure PCD (and so the set of the
structure PCDs could be filtered for set-but-unused ones).

This is not the case: in "DscBuildData.py", in method
UpdateStructuredPcds(), if "S_pcd_set" remains an empty OrderedDict(),
then it is not added to "GlobalData.gStructurePcd" *at all*, for the
current (platform, architecture) combination.

As a result, when the PCD report tries to fetch the set of structure PCDs
for the current (platform, architecture), "GlobalData.gStructurePcd" does
not return an empty OrderedDict(); instead, it raises a KeyError. Fix it
by defaulting to an empty OrderedDict(), with the get() method.

Reported-by: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1513
Fixes: 97c8f5b9e7d3136b6051a05cf056ce5ca9e79893
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>