Fix empty string environment variable support

[terryfinn]

Fix: Preserve empty string environment variables

What changed and why

This PR fixes a bug where empty string environment variables are silently ignored. The fix modifies lib/kamal/utils.rb to explicitly preserve empty strings when building Docker arguments.

The Problem

Applications often need to distinguish between undefined and empty environment variables:

# Example: Queue worker configuration
queues = ENV.fetch("QUEUES", "*").split(",")
# Undefined => "*" (all queues)
# "" => no queues (maintenance mode)
# "critical" => specific queue

With QUEUES: "" in deploy.yml, Kamal currently omits the variable entirely, causing the application to incorrectly use the default value.

The Solution

# lib/kamal/utils.rb line 11
- if value.present?
+ if value.present? || value == ""

This minimal change ensures empty strings are passed as --env VAR="" to Docker.

[Copilot]

Pull request overview

Fixes handling of environment variables configured with empty-string values so they are not silently omitted when generating Docker CLI arguments, allowing apps to distinguish between “unset” and “set to empty”.

Changes:

• Preserve empty-string values in Kamal::Utils.argumentize so env vars like EMPTY: "" are emitted as --env EMPTY="".
• Ensure shell-escaping returns a proper quoted empty string for "" values.
• Add test coverage for empty-string env values in both low-level utils and configuration env parsing.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
lib/kamal/utils.rb	Preserves "" values during argument building and emits a properly quoted empty string during shell escaping.
test/utils_test.rb	Adds a unit test asserting argumentize includes empty-string env vars as VAR="".
test/configuration/env_test.rb	Adds a configuration test ensuring clear env values preserve empty strings.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

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