FIR: return Unit from empty lambda

compiler/fir/analysis-tests/testData/resolve/expresssions/lambda.kt

@@ -13,5 +13,7 @@ fun main() {
 
     val f = { "OK" }
     val ss = f.invoke()
+
+    val empty: () -> Unit = { }
 }
 

compiler/fir/analysis-tests/testData/resolve/expresssions/lambda.txt

@@ -33,4 +33,8 @@ FILE: lambda.kt
         }
 
         lval ss: R|kotlin/String| = R|<local>/f|.R|FakeOverride<kotlin/Function0.invoke: R|kotlin/String|>|()
+        lval empty: R|() -> kotlin/Unit| = fun <anonymous>(): R|kotlin/Unit| {
+            ^ Unit
+        }
+
     }