Before this patch, `which ""` or `type ""` would say that the empty
string is `/usr/local/bin/`.
Convert callers to consistently call is_empty() on the returned string
while we're at it, to support eventually removing the is_null() String
state in the future.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
