niv motoko-base: update 91466aa5 -> b28632e4 (#4281)

## Changelog for motoko-base:
Branch: next-moc
Commits: [dfinity/motoko-base@91466aa5...b28632e4](dfinity/motoko-base@91466aa...b28632e)

* [`3d0bcedb`](dfinity/motoko-base@3d0bced) bugfix: Array.chain(as,f) incorrectly trapping when f(a) is an empty array. ([dfinity/motoko-base⁠#599](https://togithub.com/dfinity/motoko-base/issues/599))

nix/sources.json

@@ -76,10 +76,10 @@
         "homepage": null,
         "owner": "dfinity",
         "repo": "motoko-base",
-        "rev": "91466aa5e230033e4349742d755595f54dc572d2",
-        "sha256": "02z0yb2lq5yixfi262szfdy2pjpv4jp9rgpqq0hrkn7siq102ha8",
+        "rev": "b28632e47a99cfd0b31bff3d64d85cd34595c61b",
+        "sha256": "16r4l2llga0brmdl0p2mh1vfj91x411z0ipm4rbrh60h840c77ax",
         "type": "tarball",
-        "url": "https://github.com/dfinity/motoko-base/archive/91466aa5e230033e4349742d755595f54dc572d2.tar.gz",
+        "url": "https://github.com/dfinity/motoko-base/archive/b28632e47a99cfd0b31bff3d64d85cd34595c61b.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "motoko-matchers": {