fix: emit pass for empty generated classes

[kellen-miller]

Summary

• emit pass for empty generated classes
• add printer coverage for empty class bodies
• add a zero-parameter query_parameter_limit: 0 regression fixture

Root cause

When query_parameter_limit: 0 is enabled, zero-parameter queries still emit a params class. If that class has no fields, the generated Python class body is empty, which produces invalid syntax.

Validation

• go test ./internal/printer
• built the plugin locally and reproduced the zero-parameter query case in a temp directory, confirming CountBarsParams now emits pass

Closes #99.