Replace null coalescing operator with elvis operator

[jawngee]

Replace null coalescing operator with elvis operator so that empty strings return the default value. Fix for #2677

[jasonvarga]

Thank you for the PR but I don't think this is quite the right solution.

If you have a field where you explicitly define an empty string as the value, it'll use the default.

# blueprint
-
  handle: something
  field:
    type: text
    default: 'the default'

# content
---
something: ''
---

Before this change, it'll show "". After, it'll show "the default".

[jasonvarga]

Your issue is solved in c07e641