fix: custom range picker when selected range is of zero length

[AdityaHegde]

When the selected range is of zero length we end up showing end data as being before start date. Ensure we do not deduct 1ms when the selected interval is of zero length.

Note that this purely a visual fix. Actual selected range and the one used in queries does not change.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!