Remove alternateIdentifiers from XML when alternateIdentifiers parameter is blank or nil

[codycooperross]

Purpose

closes: datacite/datacite#1899

Approach

Adds safety to conversion from alternateIdentifiers param to identifiers param. Checks for the existence of alternateIdentifiers and identifiers param rather than checking if either is blank.

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Learning

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• New feature (non-breaking change which adds functionality)

• Breaking change (fix or feature that would cause existing functionality to change)

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