font: Shaper should replace 0 with 0x20 (space)

This didn't cause any real problems but it was a waste to trigger the
full font fallback system for codepoint 0.

src/font/Shaper.zig

@@ -182,7 +182,7 @@ pub const RunIterator = struct {
             // for unknown glyphs.
             const font_idx_opt = (try self.group.indexForCodepoint(
                 alloc,
-                if (cell.empty()) ' ' else cell.char,
+                if (cell.empty() or cell.char == 0) ' ' else cell.char,
                 style,
                 presentation,
             )) orelse (try self.group.indexForCodepoint(