Allow users to set examples without constraints in PROTOVALIDATE lint rule

[doriable]

Currently when a user attempts to set protovalidate examples
without constraints, buf lint returns a violation:

{"path":"...,"start_line":#,"start_column":#,"end_line"#:,"end_column":#,"type":"PROTOVALIDATE","message":"example value is specified by there are no constraints defined"}

However, it should be valid to set examples without constraints.
This removes the constraints check when validating examples.
If a user sets a valid example with no constraints, no violation is surfaced.
If a user sets an invalid example type, then this is caught in the constraint
type check.

Fixes #3830

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 12, 2025, 4:36 PM

[oliversun9]

nit: if there are no rules, could we return early here?

[doriable]

Yeah -- given that we already check that the appropriate example type is set, all other errors would be compilation errors when building the image (e.g. if I set something like string id = 1 [(buf.validate.field).string.example = 42];). So yeah, this makes sense, pushed an update!