FIR: introduce flexible types pretty rendering for no arguments case

#KT-52020 Fixed

compiler/testData/loadJava/compiledJava/modality/ModalityOfFakeOverrides.fir.txt

@@ -1,4 +1,4 @@
-public open class ModalityOfFakeOverrides : R|java/util/AbstractList<ft<kotlin/String, kotlin/String?>>| {
+public open class ModalityOfFakeOverrides : R|java/util/AbstractList<kotlin/String!>| {
     @R|java/lang/Override|() @R|org/jetbrains/annotations/NotNull|() public open operator fun get(index: R|kotlin/Int|): R|@EnhancedNullability kotlin/String|
 
     public constructor(): R|test/ModalityOfFakeOverrides|