fix: properly handle missing algorithm type (#48)

Fixing a bug that happen in npm cli when I install a library.
in Line 272 the result of parse can be null if integrity is null. Then in line 273 other is null so it breaks as below:

```
TypeError: Cannot read properties of null (reading 'pickAlgorithm')
npm verb stack     at Integrity.match (/usr/lib/node_modules/npm/node_modules/ssri/lib/index.js:273:24)
npm verb stack     at CachePolicy.satisfies (/usr/lib/node_modules/npm/node_modules/make-fetch-happen/lib/cache/policy.js:112:49)
npm verb stack     at Function.find (/usr/lib/node_modules/npm/node_modules/make-fetch-happen/lib/cache/entry.js:178:25)
npm verb stack     at async cacheFetch (/usr/lib/node_modules/npm/node_modules/make-fetch-happen/lib/cache/index.js:8:17)
npm verb stack     at async fetch (/usr/lib/node_modules/npm/node_modules/make-fetch-happen/lib/fetch.js:98:7)
```

lib/index.js

@@ -270,6 +270,9 @@ class Integrity {
   match (integrity, opts) {
     opts = ssriOpts(opts)
     const other = parse(integrity, opts)
+    if (!other) {
+      return false
+    }
     const algo = other.pickAlgorithm(opts)
     return (
       this[algo] &&

test/integrity.js

@@ -108,6 +108,7 @@ test('match()', t => {
   }, 'returns the strongest match')
   t.notOk(sri.match('sha512-foo'), 'falsy when match fails')
   t.notOk(sri.match('sha384-foo'), 'falsy when match fails')
+  t.notOk(sri.match(null), 'falsy when integrity is null')
   t.end()
 })