patch 8.2.3446: not enough tests for empty string arguments

Problem:    Not enough tests for empty string arguments.
Solution:   Add tests, fix type check. (Yegappan Lakshmanan, closes #8881)
diff --git a/runtime/doc/sign.txt b/runtime/doc/sign.txt
index 64f54db..57f7dc7 100644
--- a/runtime/doc/sign.txt
+++ b/runtime/doc/sign.txt
@@ -520,6 +520,7 @@
 		{buf} and position the cursor at sign {id} in group {group}.
 		This is similar to the |:sign-jump| command.
 
+		If {group} is an empty string, then the global group is used.
 		For the use of {buf}, see |bufname()|.
 
 		Returns the line number of the sign. Returns -1 if the