[MC-164383] Command blocks with invalid CustomName cause crash when run

The bug

If you have a command block modified to an invalid CustomName and run any command inside, the game will crash.

To reproduce

Place a command block

Run the following command:

/data merge block X Y Z {CustomName:"null"}

Put any command inside the command block and run it, for example /say hi
→ The server crashes

Stack trace

1.14.4: crash-2019-10-24_05.28.26-server.txt

Description: Executing command block

java.lang.NullPointerException: Executing command block
	at bub$1.h(SourceFile:185)
	at bgx.a(SourceFile:117)
	at bno.a(SourceFile:116)
	at bno.b(SourceFile:104)
	at bvt.a(SourceFile:265)
	at vk.b(SourceFile:565)
	at vk$$Lambda$2949/1824088252.accept(Unknown Source)
	at bie.b(SourceFile:87)
	at vk.a(SourceFile:320)
	at net.minecraft.server.MinecraftServer.b(SourceFile:835)
	at net.minecraft.server.MinecraftServer.a(SourceFile:774)
	at eac.a(SourceFile:128)
	at net.minecraft.server.MinecraftServer.run(SourceFile:642)
	at java.lang.Thread.run(Thread.java:745)