Reject an empty deps-json instead of printing a blank table

[mcocdawc]

eckit, eccodes and ecflow were all calling print-dep-table with a resolved-json:
input that does not exist on the action. GitHub only warns about an unknown
input, so the action ran on pure defaults — deps-json: '[]', own-name: '' — and
rendered an empty table on every build for months. An empty table reads as "this
package has no dependencies", not as "the caller wired nothing up".

required: true would not have caught it: the runner does not enforce required
for a composite action's inputs. But the action can reject an empty value itself,
which is what check-pr-label already does — so it now does.

• deps-json loses its '[]' default and is rejected when empty. '[]' stays the
  right value for a package with no dependencies; the difference between that and
  "nothing was passed" is the whole point.
• own-name set with an empty own-artifact-name is rejected too. That
  combination used to drop the OWN row in silence — same failure shape, one row
  smaller.

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• I have added or updated tests to verify that my changes are effective and functional.
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