fix: handle undefined values in workspace options

[dsherret]

This:

import { DenoWorkspace } from "@deno/loader";

const workspace = new DenoWorkspace({
  nodeConditions: undefined,
});

Was erroring:

error: Uncaught (in promise) Error: TypeError: Reflect.get called on non-object
  const ret = new Error(getStringFromWasm0(arg0, arg1));
              ^
    at __wbindgen_error_new (https://jsr.io/@deno/loader/0.0.7/src/lib/rs_lib.internal.js:982:15)
    at <anonymous> (wasm://wasm/00fd13ae:1:3222054)
    at <anonymous> (wasm://wasm/00fd13ae:1:3084942)
    at <anonymous> (wasm://wasm/00fd13ae:1:3154713)
    at new DenoWorkspace (https://jsr.io/@deno/loader/0.0.7/src/lib/rs_lib.internal.js:378:22)
    at new DenoWorkspace (https://jsr.io/@deno/loader/0.0.7/src/mod.ts:129:19)
    at file:///Users/marvinh/dev/denoland/esbuild-plugin-deno/foo.ts:3:19

[marvinhagemeister]

LGTM