Allow zero-length strings in x-correlator header

[bigludo7]

Problem description
Allow zero-length strings in x-correlator header
As defined here camaraproject/Commonalities#387

Expected behavior
Add/fix pattern

  headers:
    x-correlator:
      description: Correlation id for the different services
      schema:
        type: string
        pattern: ^[a-zA-Z0-9-]{0,55}$

Alternative solution

Additional context