[PATCH 3/3] MdePkg/BaseLib: Base64Decode: Add decription for RETURN_SUCCESS

["Gao, Zhichao" <zhichao.gao@intel.com>]

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1891

While the convertion Base64 ascii string is null string (the
string only contain white space would be regard as null string),
there would be no decodeable data. Set *DestinationSize to zero
and return RETURN_SUCCESS. But it is not mention in the comment
of the function. So add this decription.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
---
 MdePkg/Library/BaseLib/String.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/MdePkg/Library/BaseLib/String.c b/MdePkg/Library/BaseLib/String.c
index 7ebc2ecddd..8829d2cbbf 100644
--- a/MdePkg/Library/BaseLib/String.c
+++ b/MdePkg/Library/BaseLib/String.c
@@ -1929,7 +1929,8 @@ Base64Encode (
   @param DestinationSize   Caller is responsible for passing in buffer of at least DestinationSize.
                            Set 0 to get the size needed. Set to bytes stored on return.
 
-  @retval RETURN_SUCCESS             When binary buffer is filled in.
+  @retval RETURN_SUCCESS             When binary buffer is filled in. Or if the Base64 ascii string is empty, set
+                                     *DestinationSize to zero to indicate this case.
   @retval RETURN_INVALID_PARAMETER   If Source is NULL or DestinationSize is NULL.
   @retval RETURN_INVALID_PARAMETER   If SourceLength or DestinationSize is bigger than (MAX_ADDRESS -(UINTN)Destination ).
   @retval RETURN_INVALID_PARAMETER   If there is any invalid character in input stream.
-- 
2.21.0.windows.1