Fix to handle date fields with empty values

[wetzelj]

Added check to prevent exception when JITTER action is performed on a field with a blank value.

Description

Related issues: #189

• Added check to ensure a new jittered date value has a value before attempting to perform the replace of the field.
• Modified test dicom file to enable the creation of a new unit test to validate this change.

Checklist

• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] My code follows the style guidelines of this project

Open questions

Questions that require more discussion or to be addressed in future development:

[vsoch]

Thank you @wetzelj !

[wetzelj]

Thank you @vsoch! You're always so speedy getting these things merged in... I really appreciate it!

[wetzelj]

@vsoch... I'm irritated with myself. :( I updated the readme in this PR, but didn't update version.py. Do you want me to create a separate pull request for the version update or wait until the next PR since #186 is still open out there?

[vsoch]

Oh don’t worry I updated it locally and just need to push the change!