Update nonVoids for HTML5 void elements; accept whitespace in close tags

[JeremyIV]

Three small fixes, one commit each:

1. nonVoids list vs. the current WHATWG void-element list

• source is a void element, but it was listed as non-void, so <source src=x/> compiled to <source src="x"></source> — closing a void element is an HTML5 parse error.
• picture, slot, and search are ordinary elements but were missing from the list, so <picture/> rendered via renderSelf as a literal <picture/>, which HTML5 parses as an unclosed open tag (the slash is ignored) — everything after it ends up inside the picture element.

Fix: remove source; add picture, slot, search in alphabetical position. I also added math next to svg under the existing "since commonly used" comment — easy to drop that hunk if you'd rather not.

After the fix:

$ printf '<source src=x/>\n<picture/>\n<slot/>\n<search/>\n<math/>\n' | hxml
<source src="x"/>
<picture></picture>
<slot></slot>
<search></search>
<math></math>

2. Whitespace before > in close tags

Open tags tolerate trailing whitespace (sp <- many spaceChar before the >), but close tags didn't, so </div > failed to parse. XML's grammar allows it (ETag ::= '</' Name S? '>'). Fix: void $ many spaceChar >> char '>' in the close branch. Verified that </div >, </ >, and a close tag split across lines all parse now.

3. renderAttrVal totality

The case BLC.uncons v of Just (x, _) | ... | __ -> ... had no Nothing alternative — unreachable from current call sites (fromMaybe n v is never empty), but a latent crash. Folded the empty case into the catch-all quoting branch; no behavior change.

Verification

• cabal build clean on GHC 9.14.1 / megaparsec 9.x (only pre-existing -Wx-partial warnings).
• The test.hxml → test.html fixture comparison passes after every commit (hxml < test.hxml | diff - test.html).
• Hand-tested the examples shown above against the built binary.

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This came out of an AI review pass Jeremy ran over your repos for fun. Every finding was verified as described above, but the usual skepticism applies.

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