fix(reports): render renewal-letter premium lines as booleans

`r.netPremium` / `r.total` are strings, so an empty-string value leaked
""` into the JSX instead of rendering nothing. Wrap in Boolean() so the
guard is a real conditional.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/web/src/components/ReportRunner.tsx

@@ -595,10 +595,10 @@ function LetterLayout({ result }: { result: ReportRunResult }) {
             </div>
 
             <div className="renewal-letter-premium">
-              {r.netPremium && (
+              {Boolean(r.netPremium) && (
                 <span>Prima neta: {formatCell(r.netPremium, "money")}</span>
               )}
-              {r.total && (
+              {Boolean(r.total) && (
                 <span className="renewal-letter-total">
                   Total: {formatCell(r.total, "money")} {String(r.currency ?? "")}
                 </span>