[FIR] make FirPropertyAccessor#propertySymbol non-nullable

we need it to restore property accessor by property

^KT-54311

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2022 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -337,7 +337,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
 
         propertyAccessor.configure {
             +symbol("FirPropertyAccessorSymbol")
-            +field("propertySymbol", firPropertySymbolType, nullable = true).apply {
+            +field("propertySymbol", firPropertySymbolType).apply {
                 withBindThis = false
             }
             +booleanField("isGetter")