Refining the tests for how Bob responds to silence

The Elixir suite case doesn't current check for nil, while the Ruby test
suite demands that nil be understood as a form of silence. This adds a
test to the Elixir quite to check for nil.

It also adds a test case for a different kind of "empty" string: one
consisting solely of spaces, such as `"    "`.  Both of these test suites
will now check to see that is correctly interpreted as silence rather
than shouting (because `"   ".upcase == "   "`).