CastToNullableType: allow casting null keyword (#4907)

Fix a false positive for CastToNullableType for casting a literal `null` to a nullable type. This can be necessary when providing null as a function argument where the incoming type is parameterized. In this case using e.g. `null as? String` is less meaningful and generates a compiler warning that the cast cannot succeed.

detekt-rules-errorprone/src/main/kotlin/io/gitlab/arturbosch/detekt/rules/bugs/CastToNullableType.kt

@@ -42,6 +42,7 @@ class CastToNullableType(config: Config = Config.empty) : Rule(config) {
 
         val operationReference = expression.operationReference
         if (operationReference.getReferencedNameElementType() != KtTokens.AS_KEYWORD) return
+        if (expression.left.text == KtTokens.NULL_KEYWORD.value) return
         val nullableTypeElement = expression.right?.typeElement as? KtNullableType ?: return
 
         val message = "Use the safe cast ('as? ${nullableTypeElement.innerType?.text}')" +

detekt-rules-errorprone/src/test/kotlin/io/gitlab/arturbosch/detekt/rules/bugs/CastToNullableTypeSpec.kt

@@ -41,4 +41,15 @@ class CastToNullableTypeSpec {
         val findings = subject.compileAndLint(code)
         assertThat(findings).isEmpty()
     }
+
+    @Test
+    fun `cast null to nullable type is allowed`() {
+        val code = """
+            fun foo(a: Any?) {
+                val x = null as String?
+            }
+        """
+        val findings = subject.compileAndLint(code)
+        assertThat(findings).isEmpty()
+    }
 }