Empty string in `connections` crashes the render with a raw css-what parser error

Summary

An empty string in connections aborts the whole render with a raw CSS-parser error that names neither the component nor the pin.

<board width="30mm" height="20mm" routingDisabled>
  <resistor name="R1" resistance="1k" footprint="0402" pcbX={-8} />
  <capacitor
    name="C1"
    capacitance="1uF"
    footprint="0402"
    connections={{ pin1: "", pin2: ".R1 > .pin1" }}
  />
</board>

Actual — renderUntilSettled() throws, and no circuit JSON is produced at all:

error: Expected name, found .
  at getName (node_modules/css-what/lib/commonjs/parse.js:95:23)
  at parseSelector (node_modules/css-what/lib/commonjs/parse.js:220:17)
  at selectOne (lib/components/base-components/PrimitiveComponent/PrimitiveComponent.ts:1253:18)
  at resolveImplicitSinglePort (lib/components/primitive-components/Trace/Trace__findConnectedPorts.ts:29:47)

Two separate problems:

1. The message is not actionable. Expected name, found . is css-what internals. It doesn't say which component, which pin, or that the value was empty. Reading it, the natural guess is that .R1 > .pin1 — the one target that is actually fine — is malformed.
2. It takes the whole board down. This is a thrown exception, not a recorded error, so the sibling valid connection, the other components, and every downstream phase never run. One typo in one prop produces zero output.

Cause

NormalComponent._createTracesFromConnectionsProp() forwards every entry of props.connections straight into a Trace's to:

for (const [pinName, target] of Object.entries(props.connections)) {
  const targets = Array.isArray(target) ? target : [target]
  for (const targetPath of targets) {
    this.add(new Trace({ from: `.${this.name} > .${pinName}`, to: targetPath }))
  }
}

An empty string isn't a selector, but nothing rejects it here, so to: "" reaches selectOne and css-what throws while parsing. The parser error surfaces from a call stack several layers below the prop the user actually wrote.

Note the value survives validation upstream too — connections is typed as a record of selector strings, and "" is a string, so the props schema accepts it.

Expected

An empty or whitespace-only target should be reported as a misconfiguration that names the component and the pin, and the rest of the board should still render — matching how other bad-input cases in the codebase behave (source_component_misconfigured_error, source_ambiguous_port_reference), which record an error and continue rather than throwing.

Suggested behaviour:

<capacitor#13 name=".C1" /> has an empty connections target for pin "pin1".
Provide a selector such as ".R1 > .pin1" or "net.VCC", or remove the entry.

…with the sibling pin2: ".R1 > .pin1" connection still producing its source_trace.

Scope / judgement calls left open

• I only treat empty and whitespace-only targets as misconfigured. A non-empty but unresolvable selector (e.g. ".R9 > .pin1" where R9 doesn't exist) already has its own downstream handling, so I deliberately left that path untouched.
• The check sits in _createTracesFromConnectionsProp so it covers both the string and array forms of a connections entry.
• Rejecting "" at the props-schema level in tscircuit/props would catch it earlier, but that's a cross-repo change with a wider blast radius (it would turn today's runtime crash into a hard parse failure for anyone who currently passes "" deliberately). I kept the fix in core; happy to follow up in props if maintainers prefer that.

PR follows.

Start in lib/components/primitive-components/NormalComponent/NormalComponent.ts at _createTracesFromConnectionsProp, then read the Trace path through PrimitiveComponent.ts and Trace__findConnectedPorts.ts. Verify the empty and whitespace-only cases against the existing misconfiguration error behavior; done means rendering records a component-and-pin error, continues, and still produces the valid sibling source_trace.