Fix index dtype of empty dependency table

[hagenw]

When publishing an empty database, the changes introduced in #372 lead to an error as the conversion to a pyarrow.Table requires the correct dtype of the index of the dependency table (audb.Dependencies._df.index). The index dtype needs to be object, see

audb/audb/core/define.py

Line 64 in ce57149

DEPEND_INDEX_DTYPE = "object"

This is fixed here, by setting the index dtype explicitly, when instantiating the dependency object.
It also adjusts an existing test, to really the the initialization of the dependency table and not an already filled table, which is tested in test_call() already.

[ChristianGeng]

Ah an insidious bug.

The only thing that might be improved is the name of the test:

test___init__ does not make for a good name and the current test name test_init tries to quote the name of the method. Would maybe test_instantiation make for a good name - simply describing what it is. Apart from that, it might make sense to add a one line docstring.

Apart from that: this is a bugfix and I would simply approve it.

[hagenw]

Yes, the bug only popped up when rebasing the dev branch into main, due to a test using an empty database added in #390.

Thanks for your comments, I renamed the test to test_instantiation() and added a docstring.