Handle description of None when describing a TAP service's tables

[stargaser]

This small PR is to address an issue we have found in describing available tables from a TAP service, when a description is not provided.

To test:

import pyvo as vo
irsa_services = vo.regsearch(servicetype='table', keywords=['irsa'])
irsa_tap = irsa_services[0]
irsa_tap.service.tables.describe()

The fix in the PR is to print No description instead of raising an exception.

[cbanek]

LGTM!

[cbanek]

Since this is a user facing kind of thing, might want to add an entry to the CHANGELOG (although I'm also fine if we think this is below that bar, we can add the flag no-changelog-entry-needed)

[codecov]

Codecov Report

Merging #197 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   70.81%   70.91%   +0.09%     
==========================================
  Files          37       37              
  Lines        3910     3912       +2     
==========================================
+ Hits         2769     2774       +5     
+ Misses       1141     1138       -3

Impacted Files	Coverage Δ
pyvo/io/vosi/vodataservice.py	84.55% <100%> (+0.87%)	⬆️
pyvo/io/uws/endpoint.py	100% <0%> (ø)	⬆️

---

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 ef3dd30...6fad216. Read the comment docs.

[stargaser]

I feel like I should add a test. Looks like I should make an analog of tests/data/tables/multiple_table_descriptions.xml that is missing_table_description.xml and then add a test using that file. Does that make sense?

[stargaser]

Cross-referencing NASA-NAVO/workshop-dev#5 where we noticed the problem addressed in this PR.

[stargaser]

Huh, looks like codecov/project increased by adding the test, but codecov/patch went down. I'm puzzled.