NI: use the inferred type to check of nullable array for vararg

^KT-40555 Fixed

compiler/testData/diagnostics/tests/varargs/inferredNullableArrayAsVararg.fir.kt

@@ -0,0 +1,15 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+// FILE: Test.java
+public class Test {
+    static public void foo(String ... x) {
+
+    }
+}
+
+// FILE: test.kt
+fun <T> select(vararg x: T) = x[1]
+
+fun main(x: Array<String>?) {
+    Test.foo(*(select(arrayOf(""), null))) // no compilation errors before the fix, NPE
+}