@marchof could you please review?

Since for implementation of unsafe cast Kotlin compiler now uses generic NullPointerException I hardened filter to not run on non-Kotlin classes and to hopefully remove possibility of filtering hand-written code, i.e. so far can't imagine hand-written Kotlin code that compiles into dup instruction followed by null-check.

Merge of this PR will also allow further update of Kotlin to 1.4.20 in org.jacoco.core.test.validation.kotlin during resolution of #1126.