New J2K: Handle forced nullability correctly in nullability analysis

nj2k/testData/nullabilityAnalysis/forcedNullability.kt

@@ -0,0 +1,11 @@
+fun a(m: Map<Int, String>): String {
+    return m.get(42)
+}
+
+fun b(m: Map<Int, String>): String {
+    return m.get(42)
+}
+
+fun c() {
+    a(mapOf<Int, String>(q.to("nya")))
+}

nj2k/testData/nullabilityAnalysis/forcedNullability.kt.after

@@ -0,0 +1,11 @@
+fun a(m: Map<Int, String>): String? {
+    return m.get(42)
+}
+
+fun b(m: Map<Int, String?>): String? {
+    return m.get(42)
+}
+
+fun c() {
+    a(mapOf<Int, String>(q.to("nya")))
+}