check the range is not empty in sequential fit dialog

[jmborr]

Description of work

Added additional check so that isValidRangeFormat only executed for non-empty ranges. Otherwise it breaks all the sequential fits that do not use this input.

Addresses EWM 15793

Release notes not necessary since this is a small change, and unnoticiable by the user.

To test:

---

Reviewer

Your comments will be used as part of the gatekeeper process. Comment clearly on what you have checked and tested during your review. Provide an audit trail for any changes requested.

As per the review guidelines:

• Is the code of an acceptable quality? (Code standards/GUI standards)
• Has a thorough functional test been performed? Do the changes handle unexpected input/situations?
• Are appropriately scoped unit and/or system tests provided?
• Do the release notes conform to the guidelines and describe the changes appropriately?
• Has the relevant (user and developer) documentation been added/updated?
• If the PR author isn’t in the mantid-developers or mantid-contributors teams, add a review comment rerun ci to authorize/rerun the CI

Gatekeeper

As per the gatekeeping guidelines:

• Has a thorough first line review been conducted, including functional testing?
• At a high-level, is the code quality sufficient?
• Are the base, milestone and labels correct?

[github-actions]

Unit test results

2 863 tests   2 863 ✅  13h 11m 49s ⏱️
    1 suites      0 💤
    1 files        0 ❌

Results for commit c757e91.

♻️ This comment has been updated with latest results.

[github-actions]

System test results

808 tests   792 ✅  3h 1m 36s ⏱️
 11 suites   16 💤
 11 files      0 ❌

Results for commit c757e91.

♻️ This comment has been updated with latest results.