Fix a bug in print library "%a" should not print anything if the var argument points to a null string.

The default precision for string should be set to 0 according to MdeLib spec. 

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4634 6f19259b-4bc3-4df7-8a09-765794883524

MdePkg/Library/BasePrintLib/PrintLib.c

@@ -355,6 +355,12 @@ BasePrintLibVSPrint (
           Flags &= (~ARGUMENT_UNICODE);
           ArgumentString = "<null string>";
         }
+        //
+        // Set the default precision for string to be zero if not specified.
+        //
+        if ((Flags & PRECISION) == 0) {
+          Precision = 0;
+        }
         break;
 
       case 'c':