[FIR] Add empty instances of DeclarationCheckers and ExpressionCheckers

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/DeclarationCheckers.kt

@@ -6,6 +6,10 @@
 package org.jetbrains.kotlin.fir.analysis.checkers.declaration
 
 abstract class DeclarationCheckers {
+    companion object {
+        val EMPTY: DeclarationCheckers = object : DeclarationCheckers() {}
+    }
+
     open val declarationCheckers: List<FirBasicDeclarationChecker> = emptyList()
     open val memberDeclarationCheckers: List<FirMemberDeclarationChecker> = emptyList()
     open val constructorCheckers: List<FirConstructorChecker> = emptyList()

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/ExpressionCheckers.kt

@@ -6,6 +6,10 @@
 package org.jetbrains.kotlin.fir.analysis.checkers.expression
 
 abstract class ExpressionCheckers {
+    companion object {
+        val EMPTY: ExpressionCheckers = object : ExpressionCheckers() {}
+    }
+
     open val expressionCheckers: List<FirBasicExpresionChecker> = emptyList()
     open val qualifiedAccessCheckers: List<FirQualifiedAccessChecker> = emptyList()
     open val functionCallCheckers: List<FirFunctionCallChecker> = emptyList()