When running /tellraw in a command block, if the json contains a player selector that can't be found, the command runs anyways. This is much easier to explain with a command.

How to Reproduce:

1) Place a command block with a button on it.
2) Enter "/tellraw @a {text:'',extra:['Welcome ',

,' to the server!']}
3) Stand on the command block, and press the button. It says "Welcome, <playername> to the server!"
4) Stand away from the block, and press it again. It says "Welcome, to the server!"