[asterisk-users] ExitIf() convention?

[Jack Bates]

I want the first line of my dialplan to check and expression, and exit
from the dailplan if it is true - is there a convention for this?

My goal is to exit from the dialplan before calling Answer() if the
callerid is null. By this means I hope to work around this issue:
http://thread.gmane.org/gmane.comp.telephony.pbx.asterisk.user/217794

- I noticed that the callerid is never null for incomming calls - even
blocked numbers are "PRIVATE", while on occasions when Asterisk
incorrectly answers during an in progress conversation, the callerid is
null.

Is it correct to use:

exten => s,1,GotoIf(${ISNULL(CALLERID())}?h)

- or is there a more commonly used convention?