Add more simplifications

[timo-abele]

What's changed?

New AssertJ simplifications:

Before	After
assertThat(anOptional.isEmpty()).isFalse()	assertThat(anOptional).isNotEmpty()
assertThat(anOptional.isEmpty()).isTrue()	assertThat(anOptional).isEmpty()
assertThat(anOptional.isPresent()).isFalse()	assertThat(anOptional).isNotPresent()
assertThat(aString.contains("foo")).isFalse()	assertThat(aString).doesNotContain("foo")
assertThat(aMap.isEmpty()).isTrue()	assertThat(aMap).isEmpty()
assertThat(aMap.isEmpty()).isFalse()	assertThat(aMap).isNotEmpty()

(assertThat(anOptional.isPresent()).isTrue() is already covered)

What's your motivation?

closing #466 + adding additional transformations

Anything in particular you'd like reviewers to focus on?

Anyone you would like to review specifically?

Have you considered any alternatives or workarounds?

Any additional context

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[timtebeek]

Great addition @timo-abele; thanks a lot!