[FIR] Fix default message test failing with NPE

Return early. Otherwise, if map doesn't contain the key,
`get(factory)!!` in the next statement fails with an NPE.

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

@@ -35,6 +35,7 @@ private val messageParameterRegex = """\{\d.*?}""".toRegex()
 fun KtDiagnosticFactoryToRendererMap.verifyMessageForFactory(factory: AbstractKtDiagnosticFactory, property: KProperty<*>) = buildList {
     if (!containsKey(factory)) {
         add("No default diagnostic renderer is provided for ${property.name}")
+        return@buildList
     }
 
     val renderer = get(factory)!!