[FIR] Implement NULL_FOR_NONNULL_TYPE diagnostics, fix tests

compiler/testData/diagnostics/tests/targetedBuiltIns/mutableMapRemove.fir.kt

@@ -19,7 +19,7 @@ class KotlinMap2 : java.util.AbstractMap<String, Int>() {
 fun foo(x: MutableMap<String, Int>, y: java.util.HashMap<String, Int>, z: java.util.AbstractMap<String, Int>) {
     x.remove("", 1)
     x.remove("", <!ARGUMENT_TYPE_MISMATCH!>""<!>)
-    x.remove("", <!ARGUMENT_TYPE_MISMATCH!>null<!>)
+    x.remove("", <!NULL_FOR_NONNULL_TYPE!>null<!>)
 
     y.remove("", 1)
     y.remove("", <!ARGUMENT_TYPE_MISMATCH!>""<!>)