Address feedback on pagination examples raised in #5881.

Follow-up to #5677.

Thanks to @tafelito for actually trying out the code examples in the new
cache policies documentation!

docs/source/caching/cache-field-behavior.md

@@ -336,7 +336,8 @@ const cache = new InMemoryCache({
           merge(existing: any[], incoming: any[], { args }) {
             const merged = existing ? existing.slice(0) : [];
             // Insert the incoming elements in the right places, according to args.
-            for (let i = args.offset; i < args.offset + args.limit; ++i) {
+            const end = args.offset + Math.min(args.limit, incoming.length);
+            for (let i = args.offset; i < end; ++i) {
               merged[i] = incoming[i - args.offset];
             }
             return merged;
@@ -346,10 +347,16 @@ const cache = new InMemoryCache({
             // If we read the field before any data has been written to the
             // cache, this function will return undefined, which correctly
             // indicates that the field is missing.
-            return existing && existing.slice(
+            const page = existing && existing.slice(
               args.offset,
               args.offset + args.limit,
             );
+            // If we ask for a page outside the bounds of the existing array,
+            // page.length will be 0, and we should return undefined instead of
+            // the empty array.
+            if (page && page.length > 0) {
+              return page;
+            }
           },
         },
       },
@@ -394,10 +401,13 @@ const cache = new InMemoryCache({
               const afterIndex = existing.findIndex(
                 task => args.afterId === readField("id", task));
               if (afterIndex >= 0) {
-                return existing.slice(
+                const page = existing.slice(
                   afterIndex + 1,
                   afterIndex + 1 + args.limit,
                 );
+                if (page && page.length > 0) {
+                  return page;
+                }
               }
             }
           },