Setting a store to `null` does not update auto-subscription references

[scottgonzalez]

Describe the bug

When a store is set to null, references to $store.value continue to hold the value from the store prior to being nulled out.

Reproduction

https://svelte.dev/repl/d1c48c88ac854c5d9245b739093538de?version=3.48.0

Logs

No response

System Info

System:
    OS: macOS 11.2.3
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 24.75 MB / 32.00 GB
    Shell: 5.0.18 - /usr/local/bin/bash
  Binaries:
    Node: 16.15.0 - ~/.nave/installed/16.15.0/bin/node
    Yarn: 1.22.17 - /usr/local/bin/yarn
    npm: 8.5.5 - ~/.nave/installed/16.15.0/bin/npm
  Browsers:
    Brave Browser: 98.1.35.101
    Chrome: 101.0.4951.64
    Firefox: 94.0.2
    Safari: 14.0.3
  npmPackages:
    svelte: ^3.46.6 => 3.46.6 
    webpack: ^4.44.2 => 4.44.2

Severity

annoyance