RedfishPkg/JsonLib: Fix JsonLib build failure
VS2019 reports build failure: "warning C4701: potentially uninitialized
local variable 'Ucs2Str' used". Initial Ucs2Str to NULL at the beginning
of function.
Cc: Abner Chang <abner.chang@amd.com>
Cc: Nick Ramirez <nramirez@nvidia.com>
Signed-off-by: Nickle Wang <nicklew@nvidia.com>
Reviewed-by: Abner Chang <abner.chang@amd.com>