BIO_write_ex(): Make handing of BIO b == NULL and dlen == 0 less redundant

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15722)

crypto/bio/bio_lib.c

@@ -393,13 +393,8 @@ int BIO_write(BIO *b, const void *data, int dlen)
 
 int BIO_write_ex(BIO *b, const void *data, size_t dlen, size_t *written)
 {
-    if (dlen == 0) {
-        /* no error */
-        if (written != NULL)
-            *written = 0;
-        return 1;
-    }
-    return bio_write_intern(b, data, dlen, written) > 0;
+    return bio_write_intern(b, data, dlen, written) > 0
+        || (b != NULL && dlen == 0); /* order is important for *written */
 }
 
 int BIO_puts(BIO *b, const char *buf)