emacsPackages: don't set attributes to null

Depending on the version of the package from MELPA, nixpkgs can set
attributes like `preBuild` or `patches` to `null`. `mkDerivation`
filters out `null`, so this has the desirable effect of not
unnecessarily changing the actual derivation. But it is inconvenient
when using `overrideAttrs`: it breaks the common pattern of
`(previousAttrs.preBuild or "") + ''...''` (also used elsewhere in the
files this commit changes), which will fail to coerce `null` to a string
or list to append to.

Fix this everywhere attributes are set to null, by defaulting to `""` or
`[ ]` instead.

We cannot set the attribute conditionally, because the attributes names
returned by `overrideAttrs` cannot depend on `finalAttrs` (only the
values can).

This changes some helpers that are (indirectly) used by several
packages, but that part of the change is effectively a noop, because the
attributes involved are always set (either by the helper or on the
original derivation).

pkgs/applications/editors/emacs/elisp-packages/elpa-common-overrides.nix

@@ -116,7 +116,7 @@ in
             })
           ]
         else
-          previousAttrs.patches or null;
+          previousAttrs.patches or [ ];
       preBuild =
         if applyOrgRoamMissingPatch then
           previousAttrs.preBuild or ""
@@ -129,7 +129,7 @@ in
             popd
           ''
         else
-          previousAttrs.preBuild or null;
+          previousAttrs.preBuild or "";
     }
   );
 

pkgs/applications/editors/emacs/elisp-packages/lib-override-helper.nix

@@ -18,7 +18,7 @@ rec {
           if predicate finalAttrs previousAttrs then
             previousAttrs.packageRequires or [ ] ++ packageRequires
           else
-            previousAttrs.packageRequires or null;
+            previousAttrs.packageRequires or [ ];
       }
     );
 
@@ -89,7 +89,7 @@ rec {
           if predicate finalAttrs previousAttrs then
             previousAttrs.nativeBuildInputs or [ ] ++ [ pkgs.writableTmpDirAsHomeHook ]
           else
-            previousAttrs.nativeBuildInputs or null;
+            previousAttrs.nativeBuildInputs or [ ];
       }
     );
 }

pkgs/applications/editors/emacs/elisp-packages/melpa-packages.nix

@@ -830,7 +830,7 @@ let
                     rm --recursive --verbose etc/elisp/screenshot
                   ''
                 else
-                  previousAttrs.preBuild or null;
+                  previousAttrs.preBuild or "";
             }
           );
 
@@ -991,7 +991,7 @@ let
                       })
                     ]
                   else
-                    previousAttrs.patches or null;
+                    previousAttrs.patches or [ ];
               }
             )
           );
@@ -1055,7 +1055,7 @@ let
                   ''
                   + previousAttrs.preBuild or ""
                 else
-                  previousAttrs.preBuild or null;
+                  previousAttrs.preBuild or "";
             }
           );
 
@@ -1221,7 +1221,7 @@ let
                     rm --verbose packages/javascript/test-suppport.el
                   ''
                 else
-                  previousAttrs.preBuild or null;
+                  previousAttrs.preBuild or "";
             }
           );
 
@@ -1392,7 +1392,7 @@ let
                     })
                   ]
                 else
-                  previousAttrs.patches or null;
+                  previousAttrs.patches or [ ];
             }
           );
 
@@ -1543,7 +1543,7 @@ let
                     })
                   ]
                 else
-                  previousAttrs.patches or null;
+                  previousAttrs.patches or [ ];
             }
           );
 
@@ -1561,7 +1561,7 @@ let
                   ''
                   + previousAttrs.preBuild or ""
                 else
-                  previousAttrs.preBuild or null;
+                  previousAttrs.preBuild or "";
             }
           );