fix(runs): guard pvalue.match against null/empty bindings

[mosoriob]

Summary

• Empty-string parameter bindings (`parameter_value: ""`) fell through the truthy ternary in `params.map` to a null default, causing `pvalue.match(/^__region_geojson/)` to throw mid-render.
• LitElement kept the previous frame in the DOM (the `loading=true` spinner), so users saw an indefinite circular spinner on the Runs page even though `loading=false` had already dispatched.
• Fix: use `param.id in ensemble.bindings` membership instead of a truthy ternary, guard `.match` with `typeof pvalue === 'string'`, and coalesce null/undefined to `''` in the rendered cell.

Reproducer: a model whose `modelcatalog_parameter` has no default, with an empty-string binding from the user. Captured in `.wolf/buglog.json` as bug-067 (related: bug-063, bug-011).

Test plan

• Reload `/runs` for a thread with WAITING executions and empty-string parameter bindings — table renders, spinner clears.
• Confirm region-geojson sentinel still shows as "Region Geojson".
• No new TS diagnostics on `mint-runs.ts` lines we touched.