Remove prune-undefined behavior from em.find, isolated to findGql

[stephenh]

We've had 1 major bug plus the occasional "found before production but still surprises engineers" coming from em.find auto-pruning conditions that accidentally?surprisingly end up as undefined, i.e.:

  const approvers = await em.find(Approver, {
    user: prior.tradePartnerUser.get,
    viaTradePartnerMarketRole: tpmc.role,
    viaMarket: tpmc.market.id,
    approval: { isFinalized: false },
  });

It seems like a big enough foot gun that em.find should never prune, i.e. ^ user: undefined would be treated as "IS NULL" (or throw an error?).

We could probably also add opts:

• pruneUndefined: false is default for em.find
• pruneUndefined: true is the default for em.findGql