dhall-docs: Prelude.head: empty list

[kukimik]

Running the dhall-docs executable from the 1.42.0 release on the following file:

let isZero = \(x : Natural) -> x === 0
let errorGeneratingDocs
	= \(n : Natural) -> \(p : isZero n) ->
		True
in errorGeneratingDocs

results in:

dhall-docs: Prelude.head: empty list

A binary I compiled myself reveals some more detail:

CallStack (from HasCallStack):
  error, called at libraries/base/GHC/List.hs:1646:3 in base:GHC.List
  errorEmptyList, called at libraries/base/GHC/List.hs:85:11 in base:GHC.List
  badHead, called at libraries/base/GHC/List.hs:81:28 in base:GHC.List
  head, called at src/Dhall/Docs/CodeRenderer.hs:378:127 in dhall-docs-1.0.11-inplace:Dhall.Docs.CodeRenderer

The relevant code is:

dhall-haskell/dhall-docs/src/Dhall/Docs/CodeRenderer.hs

Lines 375 to 379 in 4d21a18

	-- calls to `head` and `last` here should never fail since `importLines`
	-- have at least one element
	let (firstImportLine, lastImportLine) = (head importLines, last importLines)
	let prefixCols = Text.take (importStartCol - currCol) firstImportLine
	let suffixCols = Text.drop (importEndCol - currCol) lastImportLine

What I found out is that the bug seems somehow related to whitespace (e.g. removing the indentation or even replacing tabs with spaces makes the error disappear).