gen3: serve the amp Ideal page for every amp model

[ptn]

The Amp block editor never showed its Ideal tab. Two thirds of amp
models were missing it entirely, and the ones that did have it got it
twice on model 0.

The editor gates amp pages on the selected model, and ends each group
of pages with a catch-all whose value list is blank — the page it shows
for every model the explicit entries don't name. We parsed that blank
list into [0] instead of into nothing, because Number('') is 0. So the
catch-all looked like a page for model 0: hidden from the other 330
models, and duplicated on the one it appeared to claim.

Blank entries are now dropped when parsing a selector value list, and a
page whose list ends up empty is treated as its group's default — used
only when no explicit page matches. Groups without a catch-all are
unaffected.