Keep tests quieter

[captn3m0]

• Asserts for one intentional warning on empty tables
• Fixes width to "None" to avoid another warning where an empty string
  results in a warning

Thought I'd start with a simpler PR before #560

This currently breaks on Python 2.7 (which will get dropped with #582 anyway)

Before:

python -m unittest discover
.............................................................................<table> is empty
'<table> </table>'
.......getSize: Not a float ''
................................................................................
----------------------------------------------------------------------
Ran 164 tests in 0.656s

OK

After:

python -m unittest discover
....................................................................................................................................................................
----------------------------------------------------------------------
Ran 164 tests in 0.655s

OK