[schema] Ignore empty strings in example value creation

[stefanprobst]

This is for #11480

For compatibility with current master, don't treat empty strings as String type, but ignore them in example value creation. Possibly revisit this for Gatsby v3.

[pieh]

I've tested this and this does work.

The problem in master is that it's not really that we filter out empty strings ... because we don't (so we can hit this problem in master - it depends on node order) - problem is that we handle date string at later stage in master.

One thing that I also found this is a bit tangential (when I was checking some contrived examples):

gatsby/packages/gatsby/src/schema/infer/example-value.js

Line 147 in 2b29d1a

type.split(`,`).forEach(t => acc.add(t))

is problematic when type is something like [[date],[string]] - is this will generate types: date] and [string (unwrapping removes brackets at the start and end but not in the middle. I workarounded it by doing this hacky stuff:

type.split(`,`).forEach(t => acc.add(t.replace(/[\[\]]/g, ``)))