Fix prepared statements in column specs

[luisfmcalado]

The following PR solves the issue with a prepared statement in the column specs. It seems that the prepared step wasn't sending the binding variables to the tablet.
Since this is my first time contributing to Vitess I hope I'm not breaking something else but the solution was to create empty binding variables for the execShard to execute properly.
It's also missing the tests because I didn't see where It should be created in the first place.

Issue: #6287

[deepthi]

hi @luisfmcalado! we welcome contributors!
Do you think you can add a testcase here?
https://github.com/vitessio/vitess/blob/master/go/test/endtoend/preparestmt/stmt_methods_test.go

[luisfmcalado]

hi @luisfmcalado! we welcome contributors!
Do you think you can add a testcase here?
https://github.com/vitessio/vitess/blob/master/go/test/endtoend/preparestmt/stmt_methods_test.go

done @deepthi. It covers the scenario that I was trying to solve.

[systay]

LGTM