[Tests] Adjust FirIdenticalCheckerHelper behavior in case .ir.txt file is absent

.ir.txt file may be absent for the following test kinds with // DUMP_IR
- codegen tests with // IGNORE_BACKEND_K1: ANY
- FirScriptCodegenTestGenerated, since it does not have K1 counterpart
- Fir*PluginBlackBoxCodegenTestGenerated, since it does not have K1 counterpart

^KT-64256 Fixed

compiler/tests-common-new/tests/org/jetbrains/kotlin/test/utils/FirIdenticalCheckerHelper.kt

@@ -20,7 +20,7 @@ abstract class FirIdenticalCheckerHelper(private val testServices: TestServices)
     abstract fun getFirFileToCompare(testDataFile: File): File?
 
     fun firAndClassicContentsAreEquals(testDataFile: File, trimLines: Boolean = false): Boolean {
-        val classicFile = getClassicFileToCompare(testDataFile) ?: return true
+        val classicFile = getClassicFileToCompare(testDataFile) ?: return false
         val firFile = getFirFileToCompare(testDataFile) ?: return true
         return contentsAreEquals(classicFile, firFile, trimLines)
     }