Fix missing margin error in guide_custom()

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Fixed bug in `guide_custom()` that would throw error with `theme_void()` 
+  (@teunbrand, #5856).
 * New helper function `ggpar()` to translate ggplot2's interpretation of 
   graphical parameters to {grid}'s interpretation (@teunbrand, #5866).
 
@@ -23,7 +25,7 @@ documentation updates.
   (@teunbrand, #5757).
 
 ## Improvements
-  
+
 * When facets coerce the faceting variables to factors, the 'ordered' class
   is dropped (@teunbrand, #5666).
 * `coord_map()` and `coord_polar()` throw informative warnings when used

R/guide-custom.R

@@ -117,7 +117,7 @@ GuideCustom <- ggproto(
     )
 
     # Add padding and background
-    gt <- gtable_add_padding(gt, elems$margin)
+    gt <- gtable_add_padding(gt, elems$margin %||% margin())
 
     gt <- gtable_add_grob(
       gt, element_grob(elems$background),

tests/testthat/test-guides.R

@@ -1273,3 +1273,23 @@ test_that("old S3 guides can be implemented", {
     )
   )
 })
+
+test_that("guide_custom can be drawn and styled", {
+
+  p <- ggplot() + guides(custom = guide_custom(
+    circleGrob(r = unit(1, "cm")),
+    title = "custom guide"
+  ))
+
+  expect_doppelganger(
+    "stylised guide_custom",
+    p + theme(legend.background = element_rect(fill = "grey50"),
+              legend.title.position = "left",
+              legend.title = element_text(angle = 90, hjust = 0.5))
+  )
+
+  expect_doppelganger(
+    "guide_custom with void theme",
+    p + theme_void()
+  )
+})