[MC-99905] Villagers spawned with no buy option breaks NBT data. Entitydata commands targeting said villager prevents chunks from saving.

Basically all necessary info is given in the title. If you summon a villager with a broken/incomplete trade, then try to target it with an entitydata command, the console will output a warning, and eventually try to, and fail to save the chunk containing said villager.

Steps to replicate:

-Summon a villager with a trade that has no cost.
-Select that villager with an Entitydata command. Eg:

/entitydata @e[type=Villager] {Invulnerable:1}

Resulting console output should be as follows:

21.03 21:13:41 [Server] Server thread/WARN Couldn't process command: 'entitydata @e[type=Villager] {Invulnerable:1}'
21.03 21:13:42 [Server] INFO Negative index in crash report handler (0/23)
21.03 21:13:42 [Server] Server thread/ERROR Failed to save chunk
21.03 21:13:42 [Server] INFO e: Saving entity NBT
21.03 21:13:42 [Server] INFO at rr.e(SourceFile:1322) ~[minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at rr.d(SourceFile:1263) ~[minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at ass.a(SourceFile:216) ~[minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at ass.a(SourceFile:102) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at lo.b(SourceFile:147) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at lo.a(SourceFile:166) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at lp.a(SourceFile:883) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at ax.a(SourceFile:36) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at j.a(SourceFile:82) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at j.a(SourceFile:65) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at mb.d(SourceFile:830) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at mb.a(SourceFile:817) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at ij.a(SourceFile:37) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at ij.a(SourceFile:9) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at fh$1.run(SourceFile:13) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_72]
21.03 21:13:42 [Server] INFO at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_72]
21.03 21:13:42 [Server] INFO at g.a(SourceFile:45) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at net.minecraft.server.MinecraftServer.D(SourceFile:597) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at la.D(SourceFile:339) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at net.minecraft.server.MinecraftServer.C(SourceFile:553) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at net.minecraft.server.MinecraftServer.run(SourceFile:457) [minecraft_server.1.9.1-pre3.jar:?]
21.03 21:13:42 [Server] INFO at java.lang.Thread.run(Thread.java:745) [?:1.8.0_72]
21.03 21:13:42 [Server] INFO Caused by: java.lang.NullPointerException