tracee: fix panic when ctrl-c after the boot

[josedonizetti]

1. Explain what the PR does

Fix #3185

check if the event is nil before continuing on all states of the pipeline, to avoid panic when a (ctrl+c seg faults) is send just after the initialization.

stages of the pipeline:

• decode -> creates the event
• queue -> already checks for nil in both stages queue and dequeue
• sort -> already checks for nil
• process -> fixed in this pr
• enrich -> fixed in this pr
• derive -> already checks for nil
• signatures -> check on the first part but second stage of finding fixed in this pr
• sink -> fixed in this pr

2. Explain how to test it

3. Other comments

[rafaeldtinoco]

LGTM. Thanks for the fix!

[geyslan]

LGTM