Fix state restoration in the notebook extension

[jtpio]

References

As reported in https://github.com/jupyterlab/retrolab/issues/358, where some commands are not available in downstream applications like RetroLab since they don't include a ILayoutRestorer.

Code changes

Fix the guard checking if the restorer is available, so it only surrounds the relevant logic.

User-facing changes

Should fix some behavior in downstream applications reusing @jupyterlab/notebook-extension:tracker as is.

Backwards-incompatible changes

None

[jupyterlab-probot]

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[fcollonval]

LGTM @jtpio

[github-actions]

Benchmark report

The execution time (in milliseconds) are grouped by test file, test type and browser.
For each case, the following values are computed: min <- [1st quartile - median - 3rd quartile] -> max.

The mean relative comparison is computed with 95% confidence.

Results table

Test file	large_code_notebook	large_md_notebook
open
chromium
actual	4123 <- [4294 - 4369 - 4413] -> 4586	2773 <- [2870 - 2903 - 2966] -> 3107
expected	4070 <- [4289 - 4336 - 4387] -> 4565	2773 <- [2863 - 2923 - 2971] -> 3109
Mean relative change	0.4% ± 0.6%	-0.2% ± 0.7%
switch-from
chromium
actual	685 <- [736 - 756 - 790] -> 1055	507 <- [536 - 567 - 1014] -> 1121
expected	701 <- [740 - 757 - 790] -> 1033	496 <- [532 - 555 - 1030] -> 1262
Mean relative change	-1.4% ± 3.2%	-2.1% ± 9.3%
switch-to
chromium
actual	345 <- [373 - 418 - 430] -> 570	248 <- [304 - 314 - 321] -> 423
expected	344 <- [389 - 415 - 430] -> 484	269 <- [305 - 314 - 325] -> 361
Mean relative change	0.1% ± 2.3%	-0.2% ± 1.7%
close
chromium
actual	668 <- [1024 - 1052 - 1073] -> 1161	495 <- [521 - 533 - 546] -> 582
expected	594 <- [1020 - 1040 - 1063] -> 1122	483 <- [521 - 532 - 541] -> 570
Mean relative change	1.6% ± 1.8%	0.5% ± 0.9%

Changes are computed with expected as reference.

[jtpio]

@meeseeksdev please backport to 3.3.x