Allow the possibility for filesystem importers with null bases (#3815)

See sass/dart-sass#2199

spec/modules.md

@@ -193,9 +193,11 @@ contract:
 
 ### Filesystem Importer
 
-A *filesystem importer* is an [importer](#importer) with an associated absolute
-`file:` URL named `base`. When a filesystem importer is invoked with a string
-named `string`:
+A *filesystem importer* is an [importer](#importer) with an optional associated
+absolute `file:` URL named `base`. When a filesystem importer is invoked with a
+string named `string`:
+
+* If `base` is null and `url` doesn't begin with `"file:"`, return null.
 
 * Let `url` be the result of [parsing `string` as a URL][parsing a URL] with
   `base` as the base URL. If this returns a failure, throw that failure.

spec/spec.md

@@ -120,10 +120,7 @@ a string.
 
 * Let `url` be the absolute `file:` URL corresponding to `path`.
 
-* Let `importer` be a [filesystem importer] with an arbitrary `base`.
-
-  > This importer will only ever be passed absolute URLs, so its base won't
-  > matter.
+* Let `importer` be a [filesystem importer] with a null `base`.
 
 * Return the result of [compiling](#compiling-a-string) `text` with `syntax`,
   `url`, `importer`, `importers`, and `load-paths`.