don't generate nullable type for val with not-null initializer

tests/testData/ast/field/file/valWithInit.kt

@@ -1,3 +1,3 @@
 open class C() {
-val f : Foo? = Foo(1, 2)
+val f : Foo = Foo(1, 2)
 }

tests/testData/ast/trait/class/interfaceWithFields.kt

@@ -1,7 +1,7 @@
 trait INode {
 class object {
-val IN : String? = "in"
-val AT : String? = "@"
+val IN : String = "in"
+val AT : String = "@"
 val COMMA_WITH_SPACE : String? = COMMA + SPACE
 }
 }

tests/testData/ast/trait/class/interfaceWithStaticFields.kt

@@ -1,7 +1,7 @@
 public trait INode {
 class object {
-public val IN : String? = "in"
-public val AT : String? = "@"
+public val IN : String = "in"
+public val AT : String = "@"
 public val COMMA_WITH_SPACE : String? = COMMA + SPACE
 }
 }