fix(nix/tag): correct no match check in discr

It uses discrDef internally, but passes `null` as the default tag name,
causing Nix to drop the attribute and return an empty attribute set if
the default case is hit. Consequently we need to check for the empty
attribute set, not `null` to figure out if there was no match found.

We can also test this behavior using `assertThrows` which was introduced
after the tag library was originally written.

Change-Id: I45adb2f9602762dfc867956323fb3f5ae4c8bd1d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6904
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

nix/tag/default.nix

@@ -78,7 +78,7 @@ let
   # Like `discrDef`, but fail if there is no match.
   discr = fs: v:
     let res = discrDef null fs v; in
-    assert lib.assertMsg (res != null)
+    assert lib.assertMsg (res != { })
       "tag.discr: No predicate found that matches ${lib.generators.toPretty {} v}";
     res;
 

nix/tag/tests.nix

@@ -4,6 +4,7 @@ let
   inherit (depot.nix.runTestsuite)
     runTestsuite
     assertEq
+    assertThrows
     it
     ;
 
@@ -50,6 +51,10 @@ let
         { int = lib.isInt; }
       ] "foo")
       { def = "foo"; })
+    (assertThrows "throws failing to match"
+      (discr [
+        { fish = x: x == 42; }
+      ] 21))
   ];
 
   match-test = it "can match things" [