Fix crash in case of empty # output

[knuesel]

Without this PR, a doctest such as

```jldoctest
nothing
# output
```

results in a BoundsError exception:

ERROR: LoadError: BoundsError: attempt to access 1-element Array{SubString{String},1} at index [2]
Stacktrace:
 [1] getindex at ./array.jl:809 [inlined]
 [2] indexed_iterate at ./tuple.jl:82 [inlined]
 [3] eval_script(::Documenter.DocTests.MutableMD2CodeBlock, ::Module, ::Dict{Symbol,Any}, ::Documenter.Documents.Document, ::String) at /home/j/.julia/packages/Documenter/b3ri2/src/DocTests.jl:242
...

[fredrikekre]

```jldoctest
nothing
# output

```

Still works I believe, but no reason to not support it without the extra empty line.

[knuesel]

Yes with an empty line it works already (but the doc says that "the amount of whitespace appearing above and below the # output line is not significant").

[mortenpi]

LGTM, thanks!