Fix for error when reading blank `nameIdentifier` property from DataCite XML

[codycooperross]

Fixes #177

Purpose

Reading from blank nameIdentifier properties in DataCite XML was causing NoMethodErrors and consequently downstream issues like datacite/datacite#1970

closes: #177

Approach

Checks for nameIdentifier content for presence before stripping.

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• [x] 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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