Update go.mod

Add string from empty is null

add IsZero to Time

Merge branch 'or-zero'

float: return error when marshaling NaN or Inf

move test badge to CircleCI

MarshalText for null.String

forgot to add this

fix godoc for IsZero

it had the old behavior from v2

JSON "" shouldn't unmarshal to a null null.String.

That's what the zero package is for.
This was a "feature", but inconsistent with the rest of the null package.

MarshalText, UnmarshalText for Time