Drop exifreader's faulty-value text; test unnamed EXIF tags

A text tag whose value lies outside the file, which exifreader reads as
"<faulty value>", no longer gives a make, model, lens model or offset
field. Tests cover such a JPEG and the backup-metadata dump keying a tag
exifreader has no name for as undefined-<tag number>.

Model: opus-5-5

src/exif.ts

@@ -58,8 +58,12 @@ export interface PhotoExif {
     gpsAltitude?: number;
 }
 
+// exifreader gives "<faulty value>" for a tag whose value lies outside the
+// file; that tag is left out like one the file lacks.
 const asString = (v: unknown): string | undefined =>
-    typeof v === "string" && v.length > 0 ? v : undefined;
+    typeof v === "string" && v.length > 0 && v !== "<faulty value>"
+        ? v
+        : undefined;
 
 const asNumber = (v: unknown): number | undefined =>
     typeof v === "number" && Number.isFinite(v) ? v : undefined;