patch: add nil check for grammer field before g.BlockSyntax call

[magiusdarrigo]

Checklist

• [x] I have read the contribution guidelines.
• [x] make test passes.
• [x] make lint passes.
• [x] New and changed code is covered by tests.
• [x] Performance improvements include benchmarks.
• [x] Changes match the documented (not just the implemented) behavior of Shopify.