Bugfix: `estimate_requests` wasn't returning a value for 1-query requests

[charlie-gallagher]

Old behavior: For requests with 1 query, estimate_requests returned NULL, which threw an error later in the aw_freeform_table function

New behavior: Now it returns the right number of requests (one) but still doesn't produce error messages

• [x] R CMD check ran with 0 errors, 0 warnings, and 0 notes
• [x] I ran a battery of queries, none threw errors