BaseTools: Optimize string concatenation

https://bugzilla.tianocore.org/show_bug.cgi?id=1288

This patch is one of build tool performance improvement
series patches.

This patch is going to use join function instead of
string += string2 statement.

Current code use string += string2 in a loop to combine
a string. while creating a string list in a loop and using
"".join(stringlist) after the loop will be much faster.

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

BaseTools/Source/Python/Workspace/InfBuildData.py

@@ -614,7 +614,9 @@ class InfBuildData(ModuleBuildClassObject):
             Instance = Record[1]
             if Instance:
                 Instance = NormPath(Instance, self._Macros)
-            RetVal[Lib] = Instance
+                RetVal[Lib] = Instance
+            else:
+                RetVal[Lib] = None
         return RetVal
 
     ## Retrieve library names (for Edk.x style of modules)