only push previes if possible

Author: ranocha

docs/make.jl

@@ -74,4 +74,7 @@ makedocs(modules = [RootedTrees],
 
 deploydocs(repo = "github.com/SciML/RootedTrees.jl",
            devbranch = "main",
-           push_preview = true)
+           # Only push previews if all the relevant environment variables are non-empty.
+           push_preview = all(!isempty,
+                              (get(ENV, "GITHUB_TOKEN", ""),
+                               get(ENV, "DOCUMENTER_KEY", ""))))