f_environ: cleanup/refactor

- use os_getenvname_at_index / os_getenv
- f_getenv: empty (*p == NUL) is not null (undefined)

src/nvim/os/env.c

@@ -45,6 +45,7 @@ void env_init(void)
 }
 
 /// Like getenv(), but returns NULL if the variable is empty.
+/// @see os_env_exists
 const char *os_getenv(const char *name)
   FUNC_ATTR_NONNULL_ALL
 {