config.h: allow all values to be NULL

might as well make things consistent instead of having certain values
that can be NULL and certain that cannot.

some of the default values are still kept in config.h for example
reasons.

config.def.h

@@ -6,13 +6,13 @@ static const int WIN_HEIGHT = 600;
 
 /* colors and font can be overwritten via X resource properties.
  * See nsxiv(1), X(7) section Resources and xrdb(1) for more information.
- *                                    X resource              default value */
+ *                                      X resource            value (NULL == default) */
 static const char *WIN_BG[]   = { "Nsxiv.window.background",   "white" };
 static const char *WIN_FG[]   = { "Nsxiv.window.foreground",   "black" };
-static const char *MARK_FG[]  = { "Nsxiv.mark.foreground",      NULL }; /* NULL means it will default to window foreground */
+static const char *MARK_FG[]  = { "Nsxiv.mark.foreground",      NULL };
 #if HAVE_LIBFONTS
-static const char *BAR_BG[]   = { "Nsxiv.bar.background",       NULL }; /* NULL means it will default to window background */
-static const char *BAR_FG[]   = { "Nsxiv.bar.foreground",       NULL }; /* NULL means it will default to window foreground */
+static const char *BAR_BG[]   = { "Nsxiv.bar.background",       NULL };
+static const char *BAR_FG[]   = { "Nsxiv.bar.foreground",       NULL };
 static const char *BAR_FONT[] = { "Nsxiv.bar.font",            "monospace-8" };
 
 /* if true, statusbar appears on top of the window */

window.c

@@ -141,8 +141,8 @@ void win_init(win_t *win)
 	res_man = XResourceManagerString(e->dpy);
 	db = res_man == NULL ? NULL : XrmGetStringDatabase(res_man);
 
-	win_bg = win_res(db, WIN_BG[0], WIN_BG[1]);
-	win_fg = win_res(db, WIN_FG[0], WIN_FG[1]);
+	win_bg = win_res(db, WIN_BG[0], WIN_BG[1] ? WIN_BG[1] : "white");
+	win_fg = win_res(db, WIN_FG[0], WIN_FG[1] ? WIN_FG[1] : "black");
 	mrk_fg = win_res(db, MARK_FG[0], MARK_FG[1] ? MARK_FG[1] : win_fg);
 	win_alloc_color(e, win_bg, &win->win_bg);
 	win_alloc_color(e, win_fg, &win->win_fg);
@@ -154,7 +154,7 @@ void win_init(win_t *win)
 	xft_alloc_color(e, bar_bg, &win->bar_bg);
 	xft_alloc_color(e, bar_fg, &win->bar_fg);
 
-	f = win_res(db, BAR_FONT[0], BAR_FONT[1]);
+	f = win_res(db, BAR_FONT[0], BAR_FONT[1] ? BAR_FONT[1] : "monospace-8");
 	win_init_font(e, f);
 
 	win->bar.l.buf = lbuf;