fix: do not add excessive space when item body is empty

[QuadnucYard]

Another compatible change. After this PR, the only breaking thing in typst 0.13 is comment placement.

The last space in the following source

#[/ foo: ]

lies in TermItem in 0.12, but lies in Markup in 0.13.
Now in both cases, the space can be stripped. So does the space before an item.

Moreover, if the item body is empty (only true for 0.13 in this case), no excessive space will be added after the marker. Previously, it would not converge.

[github-actions]

📊 Benchmark Performance Report

group                        base                                   pr
-----                        ----                                   --
attrs-cetz-manual            1.02     89.7±0.93µs        ? ?/sec    1.00     87.9±1.68µs        ? ?/sec
attrs-codly                  1.01    252.4±4.34µs        ? ?/sec    1.00    249.8±3.08µs        ? ?/sec
attrs-fletcher-diagram       1.03     55.4±0.57µs        ? ?/sec    1.00     53.9±0.67µs        ? ?/sec
attrs-fletcher-draw          1.01    157.8±2.28µs        ? ?/sec    1.00    156.3±2.51µs        ? ?/sec
attrs-tablex                 1.01   480.7±24.26µs        ? ?/sec    1.00    476.2±2.40µs        ? ?/sec
attrs-touying-core           1.01    306.4±2.98µs        ? ?/sec    1.00    304.6±3.03µs        ? ?/sec
attrs-touying-utils          1.00    164.1±1.93µs        ? ?/sec    1.00    164.1±1.83µs        ? ?/sec
attrs-undergraduate-math     1.01    122.4±4.64µs        ? ?/sec    1.00    120.8±3.50µs        ? ?/sec
pretty-cetz-manual           1.01    854.3±8.43µs        ? ?/sec    1.00    846.9±6.04µs        ? ?/sec
pretty-codly                 1.01  1935.4±16.41µs        ? ?/sec    1.00  1910.4±17.85µs        ? ?/sec
pretty-fletcher-diagram      1.01    548.9±8.01µs        ? ?/sec    1.00   544.2±11.35µs        ? ?/sec
pretty-fletcher-draw         1.01  1324.5±23.29µs        ? ?/sec    1.00  1305.9±12.83µs        ? ?/sec
pretty-tablex                1.00      3.6±0.01ms        ? ?/sec    1.00      3.6±0.02ms        ? ?/sec
pretty-touying-core          1.01      2.3±0.02ms        ? ?/sec    1.00      2.3±0.02ms        ? ?/sec
pretty-touying-utils         1.00  1294.8±10.79µs        ? ?/sec    1.00  1295.6±31.76µs        ? ?/sec
pretty-undergraduate-math    1.02   904.5±14.49µs        ? ?/sec    1.00    890.0±5.89µs        ? ?/sec

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