How do I report expenses for rental property income when the property was only rented out for part of the year? Especially for 1098 values, such as mortgage interest.

If this is still a rental property, then you can deduct the expenses.

Yes, your rental is still considered a rental even if you didn't rent it out.

From the IRS Publication 527:

Idle Property

Continue to claim a deduction for depreciation on property used in your rental activity even if it is temporarily idle (not in use). For example, if you must make repairs after a tenant moves out, you still depreciate the rental property during the time it isn’t available for rent.

Vacant rental property.

If you hold property for rental purposes, you may be able to deduct your ordinary and necessary expenses (including depreciation) for managing, conserving, or maintaining the property while the property is vacant. Enter the mortgage interest for the entire year if the property is still considered a rental. However, you can’t deduct any loss of rental income for the period the property is vacant.

If my assumption that you converted the property from personal use to a rental sometime during the tax year, then in addition to the info already provided, the below information will help to clarify a number of things that the program doesn't clarify all that well.

Rental Property Dates & Numbers That Matter.

Date of Conversion - If this was your primary residence or 2^nd home before, then this date is the day AFTER you moved out, or the date you decided to lease the property – whichever is later.
In Service Date - This is the date a renter "could" have moved in. Usually, this date is the day you put the FOR RENT sign in the front yard.
Number of days Rented - the day count for this starts from the first day a renter was contracted to move in, and/or "could" have moved in. That would be your "in service" date or after if you were asked for that. Vacant periods between renters do not count for actual days rented. Please see IRS Publication927 page 17 at https://www.irs.gov/pub/irs-pdf/p527.pdf#en_US_2020_publink1000219175 Read the “Example” in the third column.
Days of Personal Use - This number will be a big fat ZERO. Read the screen. It's asking for the number of days you lived in the property AFTER you converted it to a rental. I seriously doubt (though it is possible) that you lived in the house (or space, if renting a part of your home) as your primary residence, 2nd home, or any other personal use reasons after you converted it to a rental.
Business Use Percentage. 100%. I'll put that in words so there's no doubt I didn't make a typo here. One Hundred Percent. After you converted this property or space to rental use, it was one hundred percent business use. What you used it for prior to the date of conversion doesn't count.