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/textprop.txt b/runtime/doc/textprop.txt
index ac0d310..56f7619 100644
--- a/runtime/doc/textprop.txt
+++ b/runtime/doc/textprop.txt
@@ -160,7 +160,7 @@
 
 		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