Fix init of RemoveInstanceOfPatternMatch

[greg-at-moderne]

What's changed?

Fix the initialization of internal data structures in RemoveInstanceOfPatternMatch.

What's your motivation?

Fixing NPE observed for some non-Java sources as they "follow" Java visitors, but visitCompilationUnit(J.CompilationUnit) is never called for them:

Cannot read field "conditions" because "this.variableUsage" is null
Detail:

java.lang.NullPointerException: Cannot read field "conditions" because "this.variableUsage" is null
  org.openrewrite.staticanalysis.RemoveInstanceOfPatternMatch$RemoveInstanceOfPatternMatchVisitor.visitIdentifier(RemoveInstanceOfPatternMatch.java:99)
  org.openrewrite.staticanalysis.RemoveInstanceOfPatternMatch$RemoveInstanceOfPatternMatchVisitor.visitIdentifier(RemoveInstanceOfPatternMatch.java:63)
  org.openrewrite.staticanalysis.RemoveInstanceOfPatternMatch_RemoveInstanceOfPatternMatchVisitor_KotlinVisitor.visitIdentifier(RemoveInstanceOfPatternMatch_RemoveInstanceOfPatternMatchVisitor_KotlinVisitor.zig:119)