scripts: pack: Set correct default value for entry-point-argument.

The default value of #f led to ("..." . #f) being passed further down as an
entry point.  That is not an issue for command line invocation, since in that
code path '() was already used as a default value, but it broke docker system
test with the following error:

    building /gnu/store/dll8jj6h5pfgf6ya9skk1g1546smwbib-docker-pack.tar.gz.drv...
    tar: Removing leading `/' from member names
    Backtrace:
               6 (primitive-load "/gnu/store/g8hqzccfvn4mkm41jqs65c27gs7?")
    In ./guix/docker.scm:
        268:6  5 (build-docker-image "/gnu/store/zi2f5dfdrhviinm6jxp3cj?" ?)
    In ice-9/ports.scm:
       433:17  4 (call-with-output-file _ _ #:binary _ #:encoding _)
        476:4  3 (_ _)
    In ./guix/docker.scm:
       270:21  2 (_)
       123:40  1 (config "1996ead589ab366473d935c4d5bd0c38b998f3b299847?" ?)
    In unknown file:
               0 (list->vector ("/gnu/store/1pvqd30qi1aigjdf7s7l8v7?" . #))

    ERROR: In procedure list->vector:
    In procedure vector: Wrong type argument in position 1: ("/gnu/store/1pvqd30qi1aigjdf7s7l8v7wpvrrhfkj-profile/bin/guile" . #f)
    note: keeping build directory `/tmp/guix-build-docker-pack.tar.gz.drv-16'
    builder for `/gnu/store/dll8jj6h5pfgf6ya9skk1g1546smwbib-docker-pack.tar.gz.drv' failed with exit code 1
    build of /gnu/store/dll8jj6h5pfgf6ya9skk1g1546smwbib-docker-pack.tar.gz.drv failed
    View build log at '/var/log/guix/drvs/dl/l8jj6h5pfgf6ya9skk1g1546smwbib-docker-pack.tar.gz.drv.gz'.
    cannot build derivation `/gnu/store/dq9qk1ba0f07572m8ck3xws28x1b3rif-docker-test.drv': 1 dependencies couldn't be built
    guix build: error: build of `/gnu/store/dq9qk1ba0f07572m8ck3xws28x1b3rif-docker-test.drv' failed
    make: *** [Makefile:7044: check-system] Error 1

Breakage was introduced in 7d5168a.

* guix/scripts/pack.scm (docker-image)[entry-point-argument]: Default to '().

Change-Id: If5fc1f8bcb0981df11409636e71f2ca91b05612f

guix/scripts/pack.scm

@@ -597,7 +597,7 @@ layers."
 
             (let-keywords '#$extra-options #f
                           ((image-tag #f)
-                           (entry-point-argument #f)
+                           (entry-point-argument '())
                            (max-layers #f))
 
               (build-docker-image #$output