envp_get_val: add tests + make it more consistent with envp_get_key

In particular, this is what is now changed:
 - if line is null or empty, return null
 - if line contains no =, return an empty string

This way, if non-null is returned, that means that line was valid. If null is
returned, there is either an error with line, or malloc failed.

src/env_get_set.c

@@ -6,7 +6,7 @@
 /*   By: jguelen <marvin@42.fr>                     +#+  +:+       +#+        */
 /*                                                +#+#+#+#+#+   +#+           */
 /*   Created: 2025/02/14 18:43:38 by jguelen           #+#    #+#             */
-/*   Updated: 2025/02/18 15:41:54 by khais            ###   ########.fr       */
+/*   Updated: 2025/02/18 16:15:39 by khais            ###   ########.fr       */
 /*                                                                            */
 /* ************************************************************************** */
 
@@ -43,16 +43,27 @@ char	*envp_get_key(char *line)
 	return (key);
 }
 
-/*Designed to get the value part of an envp entry*/
+/*
+** Get the value part of a line of envp
+**
+** given that line is of the form key=value, return a string containing value,
+** which is allocated and must be freed.
+**
+** if line is null or is empty,  return NULL
+**
+** if line contains no '=', return an empty string
+**
+** if allocation fail, return NULL
+*/
 char	*envp_get_val(char *line)
 {
-	char	*value_string;
-	char	*tmp;
+	char	*val_pointer;
 
-	tmp = ft_strchr(line, '=');
-	if (!tmp)
+	if (line == NULL || line[0] == '\0')
 		return (NULL);
-	tmp++;
-	value_string = ft_strdup(tmp);
-	return (value_string);
+	val_pointer = ft_strchr(line, '=');
+	if (!val_pointer)
+		return (ft_strdup(""));
+	val_pointer++;
+	return (ft_strdup(val_pointer));
 }