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).

0 files changed, 0 insertions, 0 deletions