gen_srcs: drop data = [] / srcs = [] noise from BUILD output

[miridius]

Problem

gen_srcs always emits data = [] and srcs = [] on filegroups that have no subdir-data or no in-dir sources:

filegroup(
    name = "__clj_files",
    srcs = ["foo.clj"],
    data = [],            # noise
)

Buildifier doesn't strip empty list attrs and humans never write them, so every generated BUILD file carries a few attributes that mean nothing.

Solution

emit-bazel-kwargs now skips entries whose value is an empty collection before rendering:

(->> (:x kwargs)
     (remove (fn [[_ v]] (and (coll? v) (empty? v))))
     sort-kwargs-entries
     ...)

Same machinery, two fewer lines per filegroup output.

Verified

Regenerating a downstream consumer's BUILD files via bazel run //:gen_srcs wrote 996 BUILD files; the entire diff is:

911 -    data = [],
 85 -    srcs = [],

i.e. exactly the empty-attr lines, no semantic changes.

Related: #99 (independent fix surfaced by the same investigation).