Test: cover empty parent id

test/parent_id/dune

@@ -1,2 +1,7 @@
+(env
+ (_
+  (binaries
+   (../odoc_print/odoc_print.exe as odoc_print))))
+
 (cram
- (deps %{bin:odoc}))
+ (deps %{bin:odoc} %{bin:odoc_print}))

test/parent_id/empty_parent_id.t/run.t

@@ -0,0 +1,15 @@
+  $ echo "{0 Test}" > file.mld
+
+It is possible to have pages with no parent, even with the odoc 3 "parent-id"
+argument. In this case, an empty string is passed as argument.
+
+  $ odoc compile --parent-id "" --output-dir _odoc file.mld
+  $ ls _odoc
+  page-file.odoc
+  $ odoc_print _odoc/page-file.odoc | jq ".name"
+  {
+    "`LeafPage": [
+      "None",
+      "file"
+    ]
+  }