package/pkg-cargo: don't expand license info when empty

The cargo infrastructure automatically expands the list of licenses with
a little blurb that the licenses for vendored dependencies might be
missing in the list.

However, when a package does not define a license, the resulting text is
sub-optimal, resulting in a manifest that contains (with a leading
comma, yes):

    , vendored dependencies licenses probably not listed

In that case, pointing out some unkown licensing terms may be part of
unkonwn licensing terms is superfluous.

Instead, just let the legal infra do its job, and report an "unkown"
license altogether.

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

package/pkg-cargo.mk

@@ -96,8 +96,12 @@ $(2)_DL_ENV += BR_CARGO_MANIFEST_PATH=$$($(2)_SUBDIR)/Cargo.toml
 endif
 
 # Due to vendoring, it is pretty likely that not all licenses are
-# listed in <pkg>_LICENSE.
+# listed in <pkg>_LICENSE. If the license is unset, it is "unknown"
+# so adding unknowns to some unknown is still some other unkown,
+# so don't append the blurb in that case.
+ifneq ($$($(2)_LICENSE),)
 $(2)_LICENSE += , vendored dependencies licenses probably not listed
+endif
 
 # Note: in all the steps below, we "cd" into the build directory to
 # execute the "cargo" tool instead of passing $(@D)/Cargo.toml as the