AltProperty can sometimes be missed.

[James Jackson-South]

Umbraco can sometimes return an empty string when running IPublishContentGetPropertyValue(alias, recursive) which causes the following line to fail.

https://github.com/leekelleher/umbraco-ditto/blob/3624116a32c58dd02c773f77d9b2cbe04b5dfb53/src/Our.Umbraco.Ditto/ComponentModel/Processors/UmbracoPropertyAttribute.cs#L145

Checking for an null or empty string fixes the issue. Will push a fix soon.