Add Boolean? == const to elvis conversion and vice versa #KT-15368 Fixed

Convert elvis to equality check implemented as inspection due
code style guide

idea/testData/inspectionsLocal/nullableBooleanElvis/inIf.kt

@@ -0,0 +1,6 @@
+fun foo() {
+    var a: Boolean? = null
+    if (a <caret>?: false) {
+
+    }
+}