[FIR] Fix raw type nullability computation

Consider the nullability of the type arguments, not of the type.
This fixes a false positive ARGUMENT_TYPE_MISMATCH.

#KT-66294 Fixed

compiler/testData/diagnostics/tests/j+k/nullableRawTypeWithFlexibleArguments.kt

@@ -0,0 +1,31 @@
+// FIR_IDENTICAL
+// FILE: J.java
+package test;
+
+import org.jetbrains.annotations.Nullable;
+
+class Editor<BC extends BuildConfiguration, TARGET extends BuildTarget<BC>> {
+    public void onTargetSelected(@Nullable TARGET target) {}
+}
+
+interface BuildConfiguration {}
+interface BuildTarget<BC extends BuildConfiguration> {}
+
+class Helper extends AbstractHelper {}
+
+abstract class AbstractHelper<BC extends BuildConfiguration, TARGET extends BuildTarget<BC>> {
+    @Nullable
+    public TARGET findRunTarget() {
+        return null;
+    }
+}
+
+// FILE: test.kt
+package test
+
+private fun test(
+    editor: Editor<BuildConfiguration, BuildTarget<BuildConfiguration>>,
+    helper: Helper,
+) {
+    editor.onTargetSelected(helper.findRunTarget())
+}