Support OCT() function and fix CONV() mishandling of negative floats and empty string N

[elianddb]

MySQL Docs: https://dev.mysql.com/doc/refman/8.4/en/string-functions.html#function_oct

[jycor]

LGTM!

Add a few simple engine tests in gms/enginetest/queries/queries.go in the QueryTests slice. Something like:

{
	Query:    "select ord(i) from mytable",
	Expected: []sql.Row{...},
},

These can be run from gms/enginetest/memory_engine_test.go function TestQueriesSimple()

[elianddb]

@jycor I've completed the suggested changes, and added the test let me know if there anything else to do. Otherwise, I'll merge.