[MRG] empty zipfile collections should not raise assertion errors when loaded

[ctb]

Over in dib-lab/charcoal#171, @taylorreiter had to add checks for AssertionError in load_file_as_signatures(...). It turns out this is because empty zipfile collections evaluate to empty, and there is a better-safe-than-sorry assertion in _load_database that checks for empty databases.

This PR changes that assertion to checking for unset database objections (db is None) and adds a test.

Fixes #1545

@taylorreiter @bluegenes ready for review!

[codecov]

Codecov Report

Merging #1546 (3aa253e) into latest (e47bdb5) will increase coverage by 4.98%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1546      +/-   ##
==========================================
+ Coverage   90.26%   95.25%   +4.98%     
==========================================
  Files         126       99      -27     
  Lines       21262    17568    -3694     
  Branches     1595     1595              
==========================================
- Hits        19192    16734    -2458     
+ Misses       1843      607    -1236     
  Partials      227      227              

Flag	Coverage Δ
python	95.25% <100.00%> (+<0.01%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/sourmash_args.py	94.01% <100.00%> (ø)
tests/test_sourmash_args.py	100.00% <100.00%> (ø)
src/core/src/sketch/minhash.rs
src/core/src/ffi/utils.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/index/bigsi.rs
src/core/src/ffi/signature.rs
src/core/src/ffi/nodegraph.rs
src/core/src/cmd.rs
src/core/src/index/sbt/mod.rs
... and 19 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e47bdb5...3aa253e. Read the comment docs.

[taylorreiter]

🎉 LGTM!