Fix doc preview for dependabot PRs (#73)

* Fix doc preview for dependabot PRs

* fix parentheses

* format

Author: ranocha

docs/make.jl

@@ -94,5 +94,12 @@ makedocs(;
 deploydocs(;
     repo = "github.com/NumericalMathematics/Ariadne.jl.git",
     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", ""),
+        )
+    )
 )