falco: Plugin with field extraction capability for any event causes Falco to exit with an error in 0.32.0

Describe the bug

When attempting to run Falco 0.32.0 that is configured to load plugin in development that is a field extraction plugin that supports field extraction regardless of source, Falco will exit with the following error

Error: Plugin 'ABC' has field extraction capability but is not compatible with any enabled event source

From the documentation at https://falco.org/docs/plugins/#plugin-event-sources-and-interoperability this should be allowed.

The set of compatible event sources can also be omitted. In this case, all events will be presented to the plugin, regardless of their source.

How to reproduce it

Using configuration for Falco that contains the following plugin snippet using the JSON plugin for ease of reproduction, start Falco at version 0.32.0 and encounter the following error

plugins:
  - name: json
    library_path: libjson.so
    init_config: ""

load_plugins: [json]

$ sudo falco -c ./falco.yaml 
Sat Jun 11 11:43:43 2022: Falco version 0.32.0 (driver version 39ae7d40496793cf3d3e7890c9bbdc202263836b)
Sat Jun 11 11:43:43 2022: Falco initialized with configuration file ./falco.yaml 
Sat Jun 11 11:43:43 2022: Loading plugin (json) from file /usr/share/falco/plugins/libjson.so
Error: Plugin 'json' has field extraction capability but is not compatible with any enabled event source

Expected behaviour

The expectation is that Falco should start and the field extraction plugin is then responsible for handling any event source it is given. Using the plugin stanzas from the falco.yaml snippet from “How to reproduce it” and Falco 0.31.1, Falco starts up and is ready to start handling events.

$ sudo falco -c ./falco.yaml
Sat Jun 11 11:42:28 2022: Falco version 0.31.1 (driver version b7eb0dd65226a8dc254d228c8d950d07bf3521d2)
Sat Jun 11 11:42:28 2022: Falco initialized with configuration file ./falco.yaml 
Sat Jun 11 11:42:28 2022: Loading plugin (json) from file /usr/share/falco/plugins/libjson.so
Sat Jun 11 11:42:28 2022: Loading rules from file /etc/falco/falco_rules.yaml:
Sat Jun 11 11:42:28 2022: Loading rules from file /etc/falco/falco_rules.local.yaml:
Sat Jun 11 11:42:29 2022: Starting internal webserver, listening on port 8765

Screenshots

N/A

Environment

• Falco version:

$ falco --version
Falco version: 0.32.0
Driver version: 39ae7d40496793cf3d3e7890c9bbdc202263836b

• System info:

$ falco --support | jq .system_info
Sat Jun 11 12:02:49 2022: Falco version 0.32.0 (driver version 39ae7d40496793cf3d3e7890c9bbdc202263836b)
Sat Jun 11 12:02:49 2022: Falco initialized with configuration file /etc/falco/falco.yaml
Sat Jun 11 12:02:49 2022: Loading rules from file /etc/falco/falco_rules.yaml:
Sat Jun 11 12:02:49 2022: Loading rules from file /etc/falco/falco_rules.local.yaml:
{
  "machine": "x86_64",
  "nodename": "falco-vm",
  "release": "5.15.11-200.fc35.x86_64",
  "sysname": "Linux",
  "version": "#1 SMP Wed Dec 22 15:41:11 UTC 2021"
}

• Cloud provider or hardware configuration: N/A
• OS:

$ cat /etc/os-release 
NAME="Fedora Linux"
VERSION="35 (Thirty Five)"
ID=fedora
VERSION_ID=35
VERSION_CODENAME=""
PLATFORM_ID="platform:f35"
PRETTY_NAME="Fedora Linux 35 (Thirty Five)"
ANSI_COLOR="0;38;2;60;110;180"
LOGO=fedora-logo-icon
CPE_NAME="cpe:/o:fedoraproject:fedora:35"
HOME_URL="https://fedoraproject.org/"
DOCUMENTATION_URL="https://docs.fedoraproject.org/en-US/fedora/f35/system-administrators-guide/"
SUPPORT_URL="https://ask.fedoraproject.org/"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Fedora"
REDHAT_BUGZILLA_PRODUCT_VERSION=35
REDHAT_SUPPORT_PRODUCT="Fedora"
REDHAT_SUPPORT_PRODUCT_VERSION=35
PRIVACY_POLICY_URL="https://fedoraproject.org/wiki/Legal:PrivacyPolicy"

• Kernel:

$ uname -a
Linux falco-vm 5.15.11-200.fc35.x86_64 #1 SMP Wed Dec 22 15:41:11 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

• Installation method:

RPM

Additional context

I believe https://github.com/falcosecurity/falco/pull/1999 introduced this potential regression, as it looks to remove the check which allowed for empty event sources for field extractor plugins.