Avoid unlocking NULL

[Fan-iX]

When options(install.lock = FALSE) is set in .Rprofile, an error will be thrown when a package is successfully installed:

Error:                                           
! error in pak subprocess
Caused by error in `filelock::unlock(lockfile)`:
! `unlock()` needs a lock object, not a file name

This will abort the installation of other packages.

---

In install-binary.R, lock_cache will return NULL if getOption("install.lock")==FALSE, which is not a valid parameter for filelock::unlock :

pak/src/library/pkgdepends/R/install-binary.R

Lines 9 to 10 in 2c8de25

	lockfile <- lock_cache(lib_cache, pkg_name, getOption("install.lock"))
	on.exit(filelock::unlock(lockfile), add = TRUE)

pak/src/library/pkgdepends/R/install-utils.R

Lines 58 to 67 in 2c8de25

	lock_cache <- function(cache, pkg_name, lock = TRUE) {
	use_lock <- !identical(lock, FALSE)
	my_lock <- NULL
	if (use_lock) {
	lockfile <- file.path(cache, sprintf("%s.lock", pkg_name))
	# TODO: timeout and fail?
	my_lock <- filelock::lock(lockfile)
	}
	my_lock
	}

pak/src/library/filelock/R/package.R

Lines 170 to 175 in 2c8de25

	unlock <- function(lock) {
	if (!inherits(lock, "filelock_lock")) {
	stop("`unlock()` needs a lock object, not a file name")
	}
	.Call(c_filelock_unlock, lock)
	}

This PR adds a NULL check to avoid the error.