Don't accept null for standard enums

Test repeated standard enums to verify that they don't accept `null`
within the array list.

Author: tbkka

Sources/SwiftProtobuf/JSONDecoder.swift

@@ -501,10 +501,19 @@ internal struct JSONDecoder: Decoder {
     if scanner.skipOptionalArrayEnd() {
       return
     }
+    let maybeCustomDecodable = E.self as? _CustomJSONCodable.Type
     while true {
-      var e = E()
-      try decodeSingularEnumField(value: &e)
-      value.append(e)
+      if scanner.skipOptionalNull() {
+        if let customDecodable = maybeCustomDecodable {
+          let e = try customDecodable.decodedFromJSONNull() as! E
+          value.append(e)
+        } else {
+          throw JSONDecodingError.illegalNull
+        }
+      } else {
+        let e: E = try scanner.nextEnumValue()
+        value.append(e)
+      }
       if scanner.skipOptionalArrayEnd() {
         return
       }

Tests/LinuxMain.swift

@@ -582,6 +582,7 @@ extension Test_JSON {
         ("testRepeatedInt32", testRepeatedInt32),
         ("testRepeatedString", testRepeatedString),
         ("testRepeatedNestedMessage", testRepeatedNestedMessage),
+        ("testRepeatedEnum", testRepeatedEnum),
         ("testOneof", testOneof),
         ("testEmptyMessage", testEmptyMessage)
     ]

Tests/SwiftProtobufTests/Test_JSON.swift

@@ -876,6 +876,26 @@ class Test_JSON: XCTestCase, PBTestHelpers {
         }
     }
 
+    func testRepeatedEnum() {
+        assertJSONEncode("{\"repeatedNestedEnum\":[\"FOO\"]}") {(o: inout MessageTestType) in
+            o.repeatedNestedEnum = [.foo]
+        }
+        assertJSONEncode("{\"repeatedNestedEnum\":[\"FOO\",\"BAR\"]}") {(o: inout MessageTestType) in
+            o.repeatedNestedEnum = [.foo, .bar]
+        }
+        assertJSONDecodeSucceeds("{\"repeatedNestedEnum\":[\"FOO\",0,1,\"BAR\",-1]}") {(o:MessageTestType) in
+            o.repeatedNestedEnum == [.foo, .zero, .foo, .bar, .neg]
+        }
+        assertJSONDecodeFails("{\"repeatedNestedEnum\":[null]}")
+        assertJSONDecodeFails("{\"repeatedNestedEnum\":\"FOO\"}")
+        assertJSONDecodeFails("{\"repeatedNestedEnum\":0}")
+        assertJSONDecodeSucceeds("{\"repeatedNestedEnum\":null}") {(o:MessageTestType) in
+            o.repeatedNestedEnum == []
+        }
+        assertJSONDecodeSucceeds("{\"repeatedNestedEnum\":[]}") {(o:MessageTestType) in
+            o.repeatedNestedEnum == []
+        }
+    }
 
     // TODO: Test other repeated field types