Fix uv blocks install and maintenance tasks (#763)

Setting uv mirror to undefined causes it to set the environment var as an empty string.

src/virtualEnvironment.ts

@@ -125,7 +125,7 @@ export class VirtualEnvironment implements HasTelemetry {
       VIRTUAL_ENV: this.venvPath,
       // Empty strings are not valid values for these env vars,
       // dropping them here to avoid passing them to uv.
-      UV_PYTHON_INSTALL_MIRROR: this.pythonMirror || undefined,
+      ...(this.pythonMirror ? { UV_PYTHON_INSTALL_MIRROR: this.pythonMirror } : {}),
     };
 
     if (!this.uvPty) {