Fix do not return error on empty part returned from server, return just zero length

Author: Sergei Epatov

src/core/imap/MCIMAPSession.cpp

@@ -2050,8 +2050,9 @@ fetch_imap(mailimap * imap, bool identifier_is_uid, uint32_t identifier,
 
     mailimap_fetch_list_free(fetch_result);
 
-    if (text == NULL)
-        return MAILIMAP_ERROR_FETCH;
+    if (text == NULL) {
+        text_length = 0;
+    }
 
     * result = text;
     * result_len = text_length;