Reject empty string as value in cronvar

[gdrosos]

SUMMARY

Reject empty string values for value in the cronvar module to prevent misleading success reports in some OSes.
I also added the necessary integration test.
Fixes #10439

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

cronvar

[ansibullbot]

cc @dougluce
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[felixfontein]

Thanks for your contribution!

[felixfontein]

Restarting CI.

[felixfontein]

Hmm I guess this needs another commit, or a rebase, to get rid of the CI failures, since the CI matrix changed. I was hoping that closing + reopening updates the matrix, but apparently it does not 🤷

[gdrosos]

Thank for the immediate feedback! I rebased the branch so now the CI matrix succeeds.

[russoz]

There could be cases where the user wants to remove the existing content of the var, replacing it with "". I don't think that prohibiting empty value is a good solution for the situation. The module clearly needs to handle that case better for Ubuntu/Debian systems. What is the content it is generating that is wrong in syntax?

[gdrosos]

The module currently writes this line into the crontab:
EMPTY_VAR=
This is written exactly like that — without quotes. On Ubuntu/Debian, this causes a crontab parse error in the temporary file:

"/tmp/crontab123":0: bad minute
errors in crontab file, can't install.

So while the syntax EMPTY_VAR= is accepted on systems like Rocky and Fedora , it is rejected by Debian-based systems.

To support this use case safely across platforms, I could also modify the module to render:
EMPTY_VAR=""

instead of EMPTY_VAR= when the value is an empty string. This preserves the user's intent to clear a variable while maintaining compatibility on all platforms.