`/database/query.Execute()` should runtime if database is not specified at least once

[amylizzle]

This should succeed:

/proc/RunTest()	
	var/database/db = new("database.db")

	var/database/query/query = new("CREATE TABLE test (id int, name string, points float)")
	query.Execute(db)

This should fail:

/proc/RunTest()	
	var/database/db = new("database.db")

	var/database/query/query = new("CREATE TABLE test (id int, name string, points float)")
	query.Execute() //note the lack of arg

After the first use of Execute() with a db, any further uses of the query without the db argument should operate on the db specified in the first query use.