FIR: Adjust testData for spec tests: invoke on nullable receiver

It's a design problem, see related issue

^KT-30415

compiler/fir/analysis-tests/testData/resolve/problems/safeCallInvoke.kt

@@ -0,0 +1,13 @@
+class A {
+    operator fun invoke() {}
+}
+
+class B {
+    val bar: () -> Unit = {}
+    val foo: A = A()
+}
+
+fun main(b: B?) {
+    b?.bar() // allowed in FIR, prohibited in old FE
+    b?.foo() // allowed in FIR, prohibited in old FE
+}