If snapshot does not exist, return cy.wrap('') to allow .then() chaining

[richclingman]

When using useRelativeSnapshots, there is an error when snapshot does not exist.

Existing code is effectively

cy.writeFile(snapshotFileName, '', 'utf-8', { log: false })
  .then(evaluateLoadedSnapShots)

but cypress-io/cypress@ce8f0ee made writeFile return null to conform to other library writers.

Instead of returning result from writeFile, the fix simply does

return cy.wrap('')

so the .then() works.

This probably solves Issue #122

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