`build_stat`: don't include weight AxisValues if there is no weight axis in the fvar table

[m4rc1e]

Describe the issue
In google/fonts#5652 fontbakery is complaining that the font is missing wght STAT axisvalues. However, the font doesn't have a wght axis so this shouldn't be an error.

cc @RosaWagner

[RosaWagner]

After thinking about it, we still could have 400 in the stat table. Even with no weight axis, we still have a weight=400 so it makes sense. But the checks also raises a FAIL because 700 in not linked to 400, although 700 really do not exists.

to set it up; no axis in the source file, but a weight axis declared in the config.yaml file. Like we would do for the italic.

[vv-monsalve]

But the checks also raises a FAIL because 700 in not linked to 400, although 700 really do not exists

This is the most common case of falsy Fail for this check. It's been reported for all the Playwrite families.