formatter: guard diagnostics with no source range

[bendrucker]

Guards every formatter against an hcl.Diagnostic with a nil Subject. The JSON, SARIF, JUnit, Checkstyle, and Compact formatters all dereferenced diag.Subject unconditionally when rendering an hcl.Diagnostics error, so a diagnostic without a source range crashes the process with a nil pointer panic. HCL documents Subject as optional ("some diagnostics have no source ranges at all").

I couldn't reach this from current CLI input: every host-side diagnostic producer sets Subject, and plugin diagnostics cross the gRPC boundary as plain strings rather than hcl.Diagnostics. So this is hardening against a documented-nullable field, not a fix for a live crash. The SDK's own hclext decode does emit nil-Subject diagnostics ("Missing attribute/block"), so a future path that surfaces those host-side would hit it.

Changes

• Adds a diagRange helper in errors.go that returns a zero hcl.Range when Subject is nil.
• json.go omits the range when Subject is nil. JSONError.Range is already a pointer with omitempty for this.
• checkstyle.go, junit.go, and compact.go render through diagRange, so a missing subject yields an empty filename and zero line/column, matching how those formats already render empty issue ranges.
• sarif.go emits the result without a location when Subject is nil, mirroring the issue path that already attaches locations conditionally.

The non-nil path is byte-for-byte unchanged in every format. The default (pretty) format was already safe because HCL's DiagnosticTextWriter handles a nil Subject.

Testing

Adds a nil-Subject case to each format's table test (json, sarif, junit, checkstyle, compact). Each case panics on master and passes here.