Fix for GetJobAsync deserialization issue

[WhitWaldo]

Description

When retrieving the data from GetJobAsync, the dueTime wasn't properly deserializing. It was originally written expecting a null value if the dueTime isn't populated, but in practice this returns an empty string, so the null check doesn't catch the actual value. Updated to use string.IsNullOrWhitespace which will catch more invalid values in the property.

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #1460

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• [N/A] Created/updated tests - Going to add some E2E testing around this as it needs actual values from the runtime to validate
• [N/A] Extended the documentation