Drop malformed attributes instead of trapping on them (fixes #392)

[lukeredpath]

Fixes #392. That issue reported a try! crash in attribute materialization but was closed pending a reproducer — here is a minimal public-API one, plus a fix that removes the try! traps entirely.

Reproducer (stock 2.13.x)

let doc = try SwiftSoup.parse("<div a=\"b\"\u{0B}>hi</div>")
_ = try doc.select("[name=x]")
// Attributes.swift: Fatal error: 'try!' expression unexpectedly raised an error:
//   IllegalArgumentException, Message: "String must not be empty"

A vertical tab (0x0B) immediately after a quoted attribute value is the trigger.

Root cause

Attributes are parsed lazily into a deferred store and only materialized on first access (size(), getIgnoreCase(), the hot-attribute query-index rebuild during select(), …). A key can be non-empty as raw bytes (passing every parse-time !isEmpty guard) yet become empty after ByteSlice.trim(): ByteSlice.trim() strips 0x09…0x0D + 0x20 — this range includes 0x0B — but the HTML5 tokeniser's attribute-name delimiter set (CharacterReader.attributeNameDelims) is {\t \n \r \f space / = > NUL " ' <}, which does not include 0x0B, so 0x0B is read as a normal attribute-name character.

A 0x0B-only name therefore survives parsing, then trims to empty when materialized. appendPending/ensureMaterialized feed that empty key to the throwing Attribute/BooleanAttribute initialisers via try!, turning a recoverable validation error into a fatal, uncatchable trap.

Why only after a quoted value? Every other whitespace skip uses advanceAsciiWhitespace(), whose UInt8.isWhitespace predicate does include 0x0B, so a stray 0x0B is swallowed. Only AfterAttributeValue_quoted errors and transitions to BeforeAttributeName without advancing, leaving 0x0B to be consumed as a name by the explicit five-byte whitespace check that excludes it.

Fix

Rather than guarding the one known-bad input, this removes the try! traps from attribute materialization altogether. A throwing helper (makeMaterializedAttribute) builds the attribute, and the three call sites (appendPending, ensureMaterialized, and the single-attribute fallback) invoke it via try?, dropping any attribute that fails validation. This matches jsoup (which silently drops malformed empty-key attributes) and makes materialization resilient to any validation failure, not just the empty-key case — a third-party parser should never try! on uncontrolled input.

Tests / verification

• Adds Tests/SwiftSoupTests/PublicEmptyAttributeKeyTest.swift — boolean, valued, and <meta> selector cases, public API only. Traps before the fix; after, select returns 0 matches (attribute dropped).
• Full suite: 662 tests, 0 failures.
• Benchmark (BenchmarkProfileTest, release, SET=base,large, 500 iterations, same machine): baseline f474b11 32586.93 ms vs fixed d33de7b 32324.84 ms (−0.80%) — within run-to-run noise; the helper is @inline(__always), so codegen at the call sites is unchanged.

[aehlke]

LGTM. Thanks so much!!! Very glad to have this solved properly