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Fix IndexError in _tagmarkup_recurse when a sublist element is empty (#1172)

When a nested list element evaluates to an empty tagmarkup (e.g. `[]`),
`_tagmarkup_recurse` returns `([], [])`.  The attribute-merge block then
tries `al[0]` without first checking that `al` is non-empty, which raises
an `IndexError`.

Guard the merge with `if ral and al:` so that an empty sub-result is
simply skipped rather than crashing.

Fixes: `urwid.decompose_tagmarkup([("attr", "text"), []])` → IndexError

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