[gnc-report] Handle default_font_family being NULL

There was invalid assumption in afea6fd68. According to the following
documentation, pango_font_description_get_family can return NULL, and
g_strdup (NULL) also returns NULL.

https://docs.gtk.org/Pango/method.FontDescription.get_family.html

gnucash/report/gnc-report.c

@@ -336,7 +336,9 @@ gnc_get_default_report_font_family(void)
 
     pango_font_description_free (font_desc);
 
-    if (g_str_has_prefix (default_font_family, ".AppleSystemUIFont"))
+    if (!default_font_family)
+        return g_strdup ("Arial");
+    else if (g_str_has_prefix (default_font_family, ".AppleSystemUIFont"))
     {
         g_free (default_font_family);
         return g_strdup ("Arial");