Always use current font when looking for glyph for NULL character

[mojavelinux]

The NULL character is used as a placeholder for anchors. Although this character is never rendered, it is consulted when computing the line metrics when fallback fonts are enabled. The converter should always report that the NULL character is in the current font rather than looking for it in the fallback fonts. This will ensure that the line metrics are not impacted by a character that is not rendered.