Merge pull request #60406 from JohnAZoidberg/remove-isnull

treewide: Remove usage of isNull

pkgs/stdenv/adapters.nix

@@ -128,7 +128,7 @@ rec {
      with the following function:
 
      isFree = license: with builtins;
-       if isNull license then true
+       if license == null then true
        else if isList license then lib.all isFree license
        else license != "non-free" && license != "unfree";
 

pkgs/stdenv/darwin/default.nix

@@ -81,7 +81,7 @@ in rec {
         bintools     = { name = "${name}-binutils"; outPath = bootstrapTools; };
       };
 
-      cc = if isNull last then "/dev/null" else import ../../build-support/cc-wrapper {
+      cc = if last == null then "/dev/null" else import ../../build-support/cc-wrapper {
         inherit shell;
         inherit (last) stdenvNoCC;
 

pkgs/stdenv/linux/default.nix

@@ -96,7 +96,7 @@ let
           inherit system;
         };
 
-        cc = if isNull prevStage.gcc-unwrapped
+        cc = if prevStage.gcc-unwrapped == null
              then null
              else lib.makeOverridable (import ../../build-support/cc-wrapper) {
           name = "${name}-gcc-wrapper";