Set a fallback value for the right-divider-width in one place it was …

…missing

We already do this in most places, but this one was not covered.
Thanks to Stefano Rodighiero for bringing this matter to my attention
in issue 11: <#11>.

spacious-padding.el

@@ -197,6 +197,15 @@ Examples of valid configurations:
   '(tab-line tab-line-tab tab-line-tab-inactive)
   "Tab faces relevant to `spacious-padding-mode'.")
 
+(defun spacious-padding--get-right-divider-width (&optional no-fallback)
+  "Get the width of window divider.
+With optional NO-FALLBACK return nil if there is no value.  Else return
+a reasonable fallback value."
+  (cond
+   ((plist-get spacious-padding-widths :right-divider-width))
+   (no-fallback nil)
+   (t 30)))
+
 (defun spacious-padding--get-box-width (key &optional no-fallback)
   "Get width for :box of face represented by KEY in `spacious-padding-widths'.
 Return 4 if KEY does not have a value.  If optional NO-FALLBACK
@@ -258,8 +267,9 @@ overline."
   "Set window divider FACE to COLOR its width is greater than 1."
   (list
    face
-   `((t ,(when (> (plist-get spacious-padding-widths :right-divider-width) 1)
-           (list :background color :foreground color))))))
+   `((t
+      ,(when (> (spacious-padding--get-right-divider-width) 1)
+         (list :background color :foreground color))))))
 
 (defun spacious-padding-set-invisible-dividers (&rest _)
   "Make window dividers invisible and add padding.