Avoid unlocking NULL

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. <hr> In `install-binary.R`, `lock_cache` will return `NULL` if `getOption("install.lock")==FALSE`, which is not a valid parameter for `filelock::unlock` : https://github.com/r-lib/pak/blob/2c8de25e1206a8c78b867df4dc28408e72ab4c7c/src/library/pkgdepends/R/install-binary.R#L9-L10 https://github.com/r-lib/pak/blob/2c8de25e1206a8c78b867df4dc28408e72ab4c7c/src/library/pkgdepends/R/install-utils.R#L58-L67 https://github.com/r-lib/pak/blob/2c8de25e1206a8c78b867df4dc28408e72ab4c7c/src/library/filelock/R/package.R#L170-L175 This PR adds a NULL check to avoid the error.