Add support for empty replaced string

README.md

@@ -5078,12 +5078,14 @@ Replace in the string 'v' from the offset 'start'
 the string 'str1' by the string 'str2' once.
 Returns the offset of the replacement or STRING_FAILURE if no replacement
 was performed.
+str1 shall be a non empty string.
 
 ##### size\_t string\_replace\_all\_str (string\_t v, const char str1[], const char str2[])
 ##### size\_t string\_replace\_all (string\_t v, const string\_t str1, const string\_t str2)
 
 Replace in the string 'v' all the occurences of 
 the string 'str1' by the string 'str2'.
+str1 shall be a non empty string.
 
 ##### void string\_replace\_at (string\_t v, size\_t pos, size\_t len, const char str2[])