Fix truncate on nil value

[Neylix]

Description

Fix a bug while aggregating a summary aggregate without availability adding time.
This bug is caused when the function BeaconChain.fetch_and_aggregate_summary do not retrieve any beacon summary, by default it create an empty summary aggregate with en empty list of availability adding time.
So when aggregating this empty summary aggregate, a nil value is passed to the trunc function which cause crashes

Fixes #787

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules