JModelLegacy: Don't try to use "fast" count method for queries with union

To calculate the total number of items that a query returns, we try to use a faster method by replacing the whole SELECT part with a COUNT(*). When we are using a UNION or UNIONALL in our query, this however fails, since the number of fields of the 2 different queries when using COUNT(*) might differ. This PR excludes such queries from being treated in that special way.

Again, not easily testable, needs a maintainer to do a code review.

I will provide another PR soon, that merges the update and discover view of com_installer and which uses this feature.