Fix for sync error when the ireqs being merged have no names

[richafrank]

When no name was provided, the ireqs would have no underlying req attribute. Calling ireq.specifier while comparing them in merge, would raise an AttributeError.

Fixes #1710

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

Looks good 👍🏻 Mabe an integrational test in test_cli_sync.py instead of the unit or keep both.