[MC-140977] Villagers summoned with NBT data set have no trades unless summoned with trades defined

In 18w50a, Villagers no longer have trades by default after specifying NBT data. This was not the case in prior snapshots, when villagers had the Profession tag.

How to reproduce:

Summon a villager with VillagerData set to
```
{profession:"minecraft:toolsmith",level:5,type:"minecraft:taiga"}
```
```
{profession:"minecraft:toolsmith",level:5,type:"minecraft:taiga"}
```
Seems like a useful villager, right?
→ ❌ Right click on it, only to be dismayed that the high-ranking villager has absolutely nothing to trade.

Comments 2

tryashtar 2018-12-13T00:43:39Z

Only applies for level 4 and 5. Unless this remains the case when those are fully implemented, it's safe to assume that these trades are not done yet.

Yellow01 2018-12-13T01:12:30Z

Can confirm that levels 4 and 5 aren't fully implemented... for most villagers, that is. Farmers seem to be functional at level 4, confirming this bug is invalid.