fix: glab can not set group level variable masked or masked and hidden

Syed Saif Ali Alvirequested to merge
gitlab-community/gitlab-org/cli:syed/cli/i-7812 into main

Description

Now setting MaskedAndHidden when --hidden is explicitly passed, else leaving it nil so that omitempty drops it from the JSON body.
Applied to both the group level and project level variable creation paths

Related Issues

Resolves #7812 (closed)

How has this been tested?

Using UTs.

Screenshots (if appropriate):

Edited by Syed Saif Ali Alvi

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