fix: Fix `rglplot(edge_label = )` #2908

Job	Run time
Build source R package	4m 58s
Coverage	5m 2s
Check windows-latest (release)	17m 4s
Check macOS-latest (release)	14m 45s
Sanitizer	23m 58s
Check ubuntu-20.04 (release)	6m 33s
Check ubuntu-20.04 (devel)	5m 56s
Check ubuntu-20.04 (oldrel-1)	6m 39s
Check ubuntu-20.04 (oldrel-2)	5m 42s
Check ubuntu-20.04 (oldrel-3)	6m 9s
Check ubuntu-20.04 (oldrel-4)	6m 31s
	1h 43m 17s

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