fix: preserve empty MCP environment variables

[tandede]

Description

Preserve MCP stdio environment variables whose values are empty strings when editing an existing server configuration.

The environment parser previously required both the key and value to be non-empty after trimming. As a result, a valid entry such as EMPTY= disappeared after the configuration was converted to form values and saved again.

The parser now accepts any entry with a non-empty key while preserving an empty value. A regression test verifies that configurations containing both empty and non-empty environment variables survive a complete form conversion round trip.

Additional Notes

Validation completed:

• targeted Vitest suite: 3 tests passed
• Biome checks for both changed files
• TypeScript type checking

Screenshots

Not applicable; this change corrects configuration serialization behavior without changing the interface.

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• I grant the official edition development team the rights to freely use, modify, and distribute this code, including for commercial purposes.

• I confirm that this code is my original work, or I have obtained the appropriate authorization from the copyright holder to submit this code under these terms.

• I understand that the submitted code will be publicly released under the GPLv3 license, and may also be used in the proprietary official edition.

• I have read and agree with the above statement.

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