init: Make the arguments optional

Annotate the arguments of hdy_init() with (optional) to specify that
NULL arguments are valid. This also replaces the deprecated (allow-none)
by (nullable) to specify that the array pointed at by argv can be NULL.

src/hdy-main.c

@@ -20,10 +20,10 @@ static gint hdy_initialized = FALSE;
 
 /**
  * hdy_init:
- * @argc: (inout): Address of the <parameter>argc</parameter> parameter of
- *     your main() function (or 0 if @argv is %NULL). This will be changed if
- *     any arguments were handled.
- * @argv: (array length=argc) (inout) (allow-none): Address of the
+ * @argc: (inout) (optional): Address of the <parameter>argc</parameter>
+ *     parameter of your main() function (or 0 if @argv is %NULL). This will be
+ *     changed if any arguments were handled.
+ * @argv: (array length=argc) (inout) (nullable) (optional): Address of the
  *     <parameter>argv</parameter> parameter of main(), or %NULL. Any options
  *     understood by Handy are stripped before return.
  *