fix(stats): always initialize m_output field

[Andreagit97]

What type of PR is this?

/kind bug

Any specific area of the project related to this PR?

/area engine

What this PR does / why we need it:

When running Falco with these changes in the default config:

# todo: syscall_counters_enabled option
metrics:
  enabled: true
  interval: 5s
  # Typically, in production, you only use `output_rule` or `output_file`, but not both. 
  # However, if you have a very unique use case, you can use both together.
  output_rule: false
  output_file: ./falco_stats.json
  resource_utilization_enabled: true
  kernel_event_counters_enabled: true
  libbpf_stats_enabled: true
  convert_memory_to_mb: true
  include_empty_values: false

it crashes with a segfault. The reason was that the m_output field of the stats writer was not initialized, but we tried to use it anyway in the collection stats phase

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[FedeDP]

/approve

Makes sense, great catch!

[poiana]

LGTM label has been added.

Git tree hash: 7446a53d7586ce7915c637ec68493dd32d05b4ae

[FedeDP]

/cc @incertum

[incertum]

Thanks!

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, FedeDP, incertum

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