fix(fleet): no-issue: creating a machine with no optional fields

[dannash100]

fleet/machines/create returned 500 and created the machine anyway when every optional field was omitted: it inserts, then applies the rest of the form as an edit, and diesel refuses to build an empty update.

• Machine::update takes diesel's .optional_empty_changeset(), which also fixes machines/update sent with only machine_id.

• Application::update had the same case handled by hand, swallowing every QueryBuilderError rather than just the empty one. Now the same call, so it is one idiom and stricter.

• Guarding in create instead would mean hand-enumerating MachineUpdate's fields away from the struct.

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Fix these issues identified on the pull request. One commit per issue fixed.

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`crates/database/src/machines.rs:201`: `.optional_empty_changeset()` is called on the `Result<usize, diesel::result::Error>` returned by `.execute(db).await`, but no such method exists anywhere in the codebase (no trait definition, no import) — this will fail to compile. The sibling `Application::update` in applications.rs (~line 954-961) handles the same 'empty changeset from an all-None update' case manually via `match ... { Err(diesel::result::Error::QueryBuilderError(_)) => {}, ... }`; this PR appears to intend an extension-trait helper for the same pattern but never actually defines it.