Require non-empty measurements by default

[metachris]

Disallow empty measurements by default, to avoid misconfiguration.

Empty measurements can be allowed (for local development) by running BuilderHub with --allow-empty-measurements / ALLOW_EMPTY_MEASUREMENTS

[Copilot]

Pull Request Overview

This PR adds validation to disallow empty measurements by default, improving data quality and security. Empty measurements can now only be allowed with an explicit configuration flag for development/testing purposes.

Key changes:

• Added allowEmptyMeasurements configuration parameter to control empty measurement validation
• Implemented validation logic that rejects empty measurements maps unless explicitly allowed
• Updated CLI flags, environment variables, and documentation to support the new configuration

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ports/admin_handler.go	Added validation logic and configuration parameter for empty measurements
cmd/httpserver/main.go	Added CLI flag and environment variable support for allowing empty measurements
httpserver/e2e_test.go	Added comprehensive tests for empty measurement validation behavior
httpserver/handler_test.go	Updated constructor call to include new allowEmptyMeasurements parameter
docker/docker-compose.yaml	Added environment variable for local development convenience
go.mod	Moved golang.org/x/crypto from indirect to direct dependency
README.md	Updated documentation to reflect new validation behavior and removed empty measurement examples
Makefile	Updated development commands and removed empty measurement example from dev setup
AGENTS.md	Added development guidelines for database testing

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