NI: exclude not fixed type variables from types on which null checks should be generated

^KT-36371 Fixed

compiler/testData/codegen/box/builderInference/lackOfNullCheckOnNullableInsideBuild.kt

@@ -0,0 +1,31 @@
+// !LANGUAGE: +NewInference
+// WITH_RUNTIME
+// IGNORE_BACKEND_FIR: JVM_IR
+// Issue: KT-36371
+
+import kotlin.experimental.ExperimentalTypeInference
+
+class Foo(val string: String? = null)
+
+class Builder<T> {
+    private var resolver: ((Foo) -> T)? = null
+    fun build() = resolver!!
+
+    fun resolve(resolver: (Foo) -> T) {
+        this.resolver = resolver
+    }
+}
+
+@OptIn(ExperimentalTypeInference::class)
+fun <T> build(@BuilderInference configure: Builder<T>.() -> Unit) =
+    Builder<T>().apply(configure).build()
+
+fun box(): String {
+    val resolver = build {
+        resolve { it.string }
+    }
+
+    resolver(Foo())
+
+    return "OK"
+}