Handle missing preferredPath from the page config

[jtpio]

References

Fixes #12297 (review)

This allows alternative lab-based applications like JupyterLite to not have to provide a preferredPath in the page config, since this might not be relevant in all cases.

Code changes

Handle the case where preferredPath is missing from the page config.

User-facing changes

Should be none for most users.

Backwards-incompatible changes

None

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[echarles]

LGTM. Failing UI regression Extension Manager › Search is not relevant. Thx @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	13861 <- [14149 - 14278 - 14350] -> 14769	2566 <- [2684 - 2716 - 2746] -> 2975
expected	13845 <- [14114 - 14207 - 14306] -> 14538	2522 <- [2688 - 2721 - 2747] -> 2837
Mean relative change	0.5% ± 0.3%	0.2% ± 0.6%
switch-from
chromium
actual	565 <- [607 - 621 - 639] -> 787	424 <- [445 - 456 - 475] -> 622
expected	560 <- [603 - 613 - 626] -> 670	414 <- [444 - 452 - 461] -> 481
Mean relative change	2.8% ± 1.5%	6.0% ± 2.6%
switch-to
chromium
actual	345 <- [402 - 423 - 457] -> 513	274 <- [292 - 300 - 310] -> 365
expected	381 <- [404 - 445 - 452] -> 478	266 <- [288 - 295 - 303] -> 317
Mean relative change	0.1% ± 2.0%	2.6% ± 1.3%
close
chromium
actual	1058 <- [1125 - 1175 - 1207] -> 1246	470 <- [487 - 502 - 537] -> 585
expected	1060 <- [1078 - 1086 - 1097] -> 1147	463 <- [484 - 490 - 502] -> 527
Mean relative change	6.7% ± 1.0%	4.0% ± 1.3%

Changes are computed with expected as reference.

[jtpio]

@meeseeksdev please backport to 3.4.x