json-util: adjust to stricter types of json-stable-stringify

Version 1.2.0 of said package now comes with Typescript-style types, and
that pointed out that the result of the `stringify()` function may very
well be `undefined` instead of a string. Let's adapt to that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

lib/json-util.ts

@@ -5,9 +5,13 @@ export function fromJSON<T>(input: string): T {
 }
 
 export function toJSON<T>(input: T): string {
-    return stringify(input);
+    const result = stringify(input);
+    if (typeof result !== "string") throw new Error(`Could not convert ${input} to JSON`);
+    return result;
 }
 
 export function toPrettyJSON<T>(input: T): string {
-    return stringify(input, { space: 4 });
+    const result = stringify(input, { space: 4 });
+    if (typeof result !== "string") throw new Error(`Could not convert ${input} to JSON`);
+    return result;
 }