Handle Tectonic defaults

[fridtjof]

The now added tectonic support handles a missing inputs entry in the config as if no files should be considered.

This is not correct in two ways:

By default, if no inputs are explicitly specified, tectonic will fall back to a value of ["_preamble.tex", "index.tex", "_postamble.tex"]

Further, in the currently released (stable?) version 0.15.0, inputs does not exist (it was added 4 days after that release). Instead, there were separate entries for preamble, index and postamble: https://github.com/tectonic-typesetting/tectonic/blob/8456df1fa04f7501e91e64ae021723e5dce8d751/docs/src/ref/tectonic-toml.md
Tectonic 0.15.0 also refuses to run with inputs specified.

These dedicated entries were removed completely in favor of inputs, but the defaults are the same in both cases.

[PHPirates]

Thanks, I didn't realise there was a default. I had expected Tectonic to create a new release at some point, but if they don't I guess we just have to check the Tectonic version during runtime.