Fix isSameAs(null) compile error from null reference comparisons

[timtebeek]

What

assertThat(null == a).isEqualTo(true) was being rewritten to the uncompilable assertThat(null).isSameAs(a).

The transformation comes from the external picnic AssertJObjectRulesRecipes (error-prone-support Refaster rules), wired into the Assertj composite. Its Refaster template binds the left operand of a == b as the actual value, so when the left side is the null literal you get assertThat(null), which doesn't compile. (Refaster itself avoids this via a custom heuristic that isn't ported to OpenRewrite.)

How

Adds SimplifyAssertJNullRelatedAssertion, which runs before the picnic rules in assertj.yml and converges null reference comparisons to the dedicated, more expressive assertion:

before	after
assertThat(null == a).isEqualTo(true)	assertThat(a).isNull()
assertThat(a == null).isTrue()	assertThat(a).isNull()
assertThat(a != null).isTrue()	assertThat(a).isNotNull()
assertThat(a == null).isFalse()	assertThat(a).isNotNull()

It mirrors the existing SimplifyAssertJInstanceOfAssertion: matches isTrue()/isFalse() and isEqualTo(true|false) (so it fires before the picnic boolean+object rules in the same cycle), unwraps parentheses and ! negations, and only intervenes when one operand is the null literal — genuine x == y still becomes isSameAs(y) via the picnic rule.

Tests

• New SimplifyAssertJNullRelatedAssertionTest covering both operand orders, !=/isFalse, isEqualTo(boolean), parenthesized/negated, fully-qualified assertThat, and the no-change cases.

• New @Issue(868) integration test through the full Assertj composite in AssertJBestPracticesTest, plus updated SonarDedicatedAssertions expectations (x == null now yields isNull() rather than isSameAs(null)).

• Regenerated recipes.csv.

• Fixes AssertJ recipes should not change assertThat(null== a).isEqualTo(true) to assertThat(null).isSameAs(a) #868