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)

runtime/doc/sign.txt

@@ -520,6 +520,7 @@ sign_jump({id}, {group}, {buf})
 		{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

runtime/doc/textprop.txt

@@ -160,7 +160,7 @@ prop_add({lnum}, {col}, {props})
 
 		Can also be used as a |method|: >
 			GetLnum()->prop_add(col, props)
-
+<
 						*prop_add_list()*
 prop_add_list({props}, [[{lnum}, {col}, {end-lnum}, {end-col}], ...])
 		Similar to prop_add(), but attaches a text property at