[upload] Ignore metadata when passing empty string

[uilianries]

• Added the support to ignore metadata upload when passing --metadata="" to conan upload command.
• Mixing no upload for metadata with a pattern for upload is considered invalid and user error (e.g `--metadata="" --metadata="logs/*")
• Add 2 new tests to validate metadata ignored upload
• Replaced duplicated pattern for package create and metadata create by regular methods in metadata upload test file.

Changelog: Feature: Ignore metadata upload by passing --metadata=""
Docs: conan-io/docs#3436

closes #15006

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