More validation fixes.

src/Squidex.Domain.Apps.Core.Operations/ValidateContent/JsonValueConverter.cs

@@ -179,7 +179,11 @@ namespace Squidex.Domain.Apps.Core.ValidateContent
 
                 foreach (var item in array)
                 {
-                    if (item is JsonScalar<string> s)
+                    if (item is JsonNull n)
+                    {
+                        result.Add(null);
+                    }
+                    else if (item is JsonScalar<string> s)
                     {
                         result.Add(s.Value);
                     }

src/Squidex.Domain.Apps.Core.Operations/ValidateContent/ValidatorsFactory.cs

@@ -171,7 +171,7 @@ namespace Squidex.Domain.Apps.Core.ValidateContent
                 yield return new CollectionItemValidator(new AllowedValuesValidator<string>(field.Properties.AllowedValues));
             }
 
-            yield return new CollectionItemValidator(new RequiredStringValidator());
+            yield return new CollectionItemValidator(new RequiredStringValidator(true));
         }
     }
 }

tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/StringFieldTests.cs

@@ -39,7 +39,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         }
 
         [Fact]
-        public async Task Should_add_error_if_string_is_required()
+        public async Task Should_add_error_if_string_is_required_but_null()
         {
             var sut = Field(new StringFieldProperties { IsRequired = true });
 
@@ -50,7 +50,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         }
 
         [Fact]
-        public async Task Should_add_error_if_string_is_required_and_empty_string_is_passed()
+        public async Task Should_add_error_if_string_is_required_but_empty()
         {
             var sut = Field(new StringFieldProperties { IsRequired = true });
 

tests/Squidex.Domain.Apps.Core.Tests/Operations/ValidateContent/TagsFieldTests.cs

@@ -59,7 +59,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         }
 
         [Fact]
-        public async Task Should_add_error_if_tags_are_required_and_null()
+        public async Task Should_add_error_if_tags_are_required_but_null()
         {
             var sut = Field(new TagsFieldProperties { IsRequired = true });
 
@@ -70,7 +70,7 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
         }
 
         [Fact]
-        public async Task Should_add_error_if_tags_are_required_and_empty()
+        public async Task Should_add_error_if_tags_are_required_but_empty()
         {
             var sut = Field(new TagsFieldProperties { IsRequired = true });
 
@@ -80,6 +80,28 @@ namespace Squidex.Domain.Apps.Core.Operations.ValidateContent
                 new[] { "Field is required." });
         }
 
+        [Fact]
+        public async Task Should_add_error_if_tag_value_is_null()
+        {
+            var sut = Field(new TagsFieldProperties { IsRequired = true });
+
+            await sut.ValidateAsync(JsonValue.Array(JsonValue.Null), errors);
+
+            errors.Should().BeEquivalentTo(
+                new[] { "[1]: Field is required." });
+        }
+
+        [Fact]
+        public async Task Should_add_error_if_tag_value_is_empty()
+        {
+            var sut = Field(new TagsFieldProperties { IsRequired = true });
+
+            await sut.ValidateAsync(CreateValue(string.Empty), errors);
+
+            errors.Should().BeEquivalentTo(
+                new[] { "[1]: Field is required." });
+        }
+
         [Fact]
         public async Task Should_add_error_if_value_is_not_valid()
         {