Add test on safecall of function with returns(null) contract

Currently with non-desired testdata, fix incoming in the next commit.

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/safecallAndReturnsNull.kt

@@ -0,0 +1,17 @@
+// !LANGUAGE: +AllowContractsForCustomFunctions +UseReturnsEffect
+// !USE_EXPERIMENTAL: kotlin.contracts.ExperimentalContracts
+
+import kotlin.contracts.*
+
+fun Any.nullWhenString(): Any? {
+    contract {
+        returns(null) implies (this@nullWhenString is String)
+    }
+    return if (this is String) null else this
+}
+
+fun test(x: Int?) {
+    if (x?.nullWhenString() == null) {
+        <!DEBUG_INFO_SMARTCAST!>x<!>.length
+    }
+}

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/safecallAndReturnsNull.txt

@@ -0,0 +1,5 @@
+package
+
+public fun test(/*0*/ x: kotlin.Int?): kotlin.Unit
+public fun kotlin.Any.nullWhenString(): kotlin.Any?
+    Returns(NULL) -> <this> is String