fix: prevent TypeError when ws enabled but server is undefined

[bjnewman]

Closes #143

Hey! While working on #1162 (dropping Node 14/16/18), we ran into this issue and figured we'd fix it too.

The bug

When ws: true is enabled but the server object is undefined, trying to call catchUpgradeRequest(server) crashes with:

TypeError: Cannot read property 'on' of null at catchUpgradeRequest

Happens with spdy/HTTP2, Bun, Deno, and some SSL setups.

The fix

Simple guard check before calling catchUpgradeRequest:

if (this.proxyOptions.ws === true && server) {
  this.catchUpgradeRequest(server);
}

Tests

Added 6 edge case tests:

• Server is undefined
• Server is null
• Multiple requests without server
• ws:false with missing server
• Matching path scenarios
• Websockets still work when server exists

All 193 tests passing.

Thanks for maintaining this project!