bpo-43125: Fix: return expected type (str), not original value (bytes) in email/base64mime.py::body_encode (GH-24476)

diff --git a/Lib/email/base64mime.py b/Lib/email/base64mime.py
index 17f0818..a7cc373 100644
--- a/Lib/email/base64mime.py
+++ b/Lib/email/base64mime.py
@@ -84,7 +84,7 @@ def body_encode(s, maxlinelen=76, eol=NL):
     in an email.
     """
     if not s:
-        return s
+        return ""
 
     encvec = []
     max_unencoded = maxlinelen * 3 // 4