add empty! in read! method

[jonathanBieler]

Most BioJulia packages use

reader = X.Reader(GzipDecompressorStream(open(file)))
record = X.Record()
read!(reader, record)

to read in-place, while GFF3 requires to call empty! before reading. This change makes it follow the same interface as the rest of the ecosystem.

I tried to reproduce the issue I ran into when I was not calling empty! but all the test files where already working. I still kept the additional tests.

Problematic file was this one :

https://ftp.ebi.ac.uk/pub/databases/gencode/Gencode_human/release_26/gencode.v26.basic.annotation.gff3.gz
https://www.gencodegenes.org/human/release_26.html

But it now works.

Types of changes

This PR implements the following changes:
(Please tick any or all of the following that are applicable)

• ✨ New feature (A non-breaking change which adds functionality).
• 🐛 Bug fix (A non-breaking change, which fixes an issue).
• 💥 Breaking change (fix or feature that would cause existing functionality to change).

[kescobo]

LGTM! Thanks for taking this on 👍

[kescobo]

Segfault on nightly seems unrelated ?

[jakobnissen]

Looks good to me too, thanks for doing this!