Add ConversionTest assertions to increase coverage

[TheRealHaui]

Added new Unit Tests to increase code coverage.

[garydgregory]

Hello @TheRealHaui
Have you thought about making the same changes for the whole class?

[TheRealHaui]

Hi @garydgregory,
to be honest ... yes of course!
And to be honest too ... I saw that this then is going to take me even longer than I thought when I already thought this is going to take me even longer than I anticipated initially.
Then I thought: Well, some new tests are better than no new tests even if it is not the entirely full perfect solution for the single class I was working one.

However, to say it clear I really, really appreciate and value your fast constructive response - here and in the other Pull Request apache/commons-text#719 too!

So, long story short: Stay tuned. I will do that now immediately.

Thanks!

[TheRealHaui]

@garydgregory,
here we go.
Additional Unit Tests for the class added.
Further, checkstyle does not complain too.