MatchOptions.IgnoreFields() throws SnapshotFieldException

[lamLeX]

Describe the bug
I'm trying to set the ignorefields with this syntax

result.MatchSnapshot(matchOptions=>matchOptions.IgnoreFields("$.[?(@.Result != null)].Result.Id"))

it failed on certain test cases

This case passes

[
  {
    "Error": "",
    "IsEmpty": false,
    "Result": {
      "Id": "cabcf393-e26e-4adb-af69-7125ca6774e8",
      // additional properties here abbreviated for concise
    },
    "ReplyCase": "Result"
  },
  {
    "Error": "",
    "IsEmpty": true,
    "Result": null,
    "ReplyCase": "IsEmpty"
  }
]

This case also passes

[
  {
    "Error": "",
    "IsEmpty": false,
    "Result": {
      "Id": "a9039bbb-3e65-485b-85ae-b752a8169401",
      // additional properties here abbreviated for concise
    },
    "ReplyCase": "Result"
  },
  {
    "Error": "",
    "IsEmpty": false,
    "Result": {
      "Id": "58e3b1fa-3d2b-4521-a241-6f58948ab411",
      // additional properties here abbreviated for concise
    },
    "ReplyCase": "Result"
  }
]

However, this case will fail

[
  {
    "Error": "",
    "IsEmpty": true,
    "Result": null,
    "ReplyCase": "IsEmpty"
  }
]

with this exception

Snapshooter.Exceptions.SnapshotFieldException
The fields of '$.[?(@.Result != null)].Result.Id' of the compare context caused an error. No fields of the path '$.[?(@.Result != null)].Result.Id' could not be found.

Expected behavior
all presented test cases should pass

[lamLeX]

Should we relax this check

snapshooter/src/Snapshooter/FieldOption.cs

Lines 99 to 103 in fc6b18a

	if (fields == null || fields.Count() == 0)
	{
	throw new SnapshotFieldException(
	$"No fields of the path '{fieldPath}' could not be found.");
	}

And return Empty array instead?

T[] fieldValues = (fields?.Select(f => ConvertToType<T>(f)) ?? Enumerable.Empty<T>).ToArray();

[woeterman94]

Is it possible to ignore fields if they don't exist? We have some fields that are not always in the result.

[thomasrosdahl]

It's also possible to add an extension method like this:

public static class MatchOptionsExtensions
    {
        public static MatchOptions IgnoreFieldsWhen(this MatchOptions matchOptions, bool condition, string fieldsPath)
        {
            if (condition)
            {
                matchOptions.IgnoreFields(fieldsPath);
            }

            return matchOptions;
        }
    }

Then .IgnoreFieldsWhen(data.Result != null, "$.Result.Id")