[Wasm] Don't wrap single null expression into composite in WasmNullCoercingLowering

Otherwise, it breaks check/assert inside DeclarationGenerator::visitField (DeclarationGenerator.kt:458)

#KT-56166 In-Progress

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/lower/WasmNullCoercionLowering.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2023 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.
  */
 
@@ -7,9 +7,9 @@ package org.jetbrains.kotlin.backend.wasm.lower
 
 import org.jetbrains.kotlin.backend.wasm.WasmBackendContext
 import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
-import org.jetbrains.kotlin.ir.backend.js.JsCommonBackendContext
 import org.jetbrains.kotlin.ir.backend.js.ir.JsIrBuilder
 import org.jetbrains.kotlin.ir.backend.js.lower.AbstractValueUsageLowering
+import org.jetbrains.kotlin.ir.expressions.IrConst
 import org.jetbrains.kotlin.ir.expressions.IrConstKind
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.expressions.impl.IrConstImpl
@@ -27,10 +27,13 @@ import org.jetbrains.kotlin.ir.types.makeNotNull
 class WasmNullCoercingLowering(private val contextx: WasmBackendContext) : AbstractValueUsageLowering(contextx) {
     override fun IrExpression.useExpressionAsType(actualType: IrType, expectedType: IrType): IrExpression =
         if (actualType.makeNotNull().isNothing() && actualType.isNullable() && !expectedType.makeNotNull().isNothing() && expectedType != contextx.wasmSymbols.voidType)
-            JsIrBuilder.buildComposite(
-                expectedType,
-                listOf(this, IrConstImpl(UNDEFINED_OFFSET, UNDEFINED_OFFSET, expectedType, IrConstKind.Null, null))
-            )
+            if (this is IrConst<*> && this.kind == IrConstKind.Null)
+                IrConstImpl(this.startOffset, this.endOffset, expectedType, IrConstKind.Null, null)
+            else
+                JsIrBuilder.buildComposite(
+                    expectedType,
+                    listOf(this, IrConstImpl(UNDEFINED_OFFSET, UNDEFINED_OFFSET, expectedType, IrConstKind.Null, null))
+                )
         else
             this
 }