Claude Desktop: substitute `user_config` options with no value

[pl4nty]

Describe the bug
In Claude Desktop, user_config options are only substituted if they have a value (user-provided or via default). If a value isn't provided, the raw string is passed to the server, potentially causing parser errors.

To Reproduce
Steps to reproduce the behavior:

1. Create a manifest with user_config, like

{
  "server": {
    "mcp_config": {
      ...
      "env": {
        "AZURE_TENANT_ID": "${user_config.azure_tenant_id}",
      }
    }
  },
  "user_config": {
    "azure_tenant_id": {
      "type": "string",
      "title": "Entra Tenant ID",
      "description": "Entra tenant ID. Use this to authenticate against a specific tenant. Example: 00000000-0000-0000-0000-000000000000"
    }
  }
}

2. Start the server without filling azure_tenant_id
3. Observe the server crash, when it attempts to parse the string ${user_config.azure_tenant_id} as a guid

Expected behavior
The server starts with AZURE_TENANT_ID="" or (ideally) no AZURE_TENANT_ID at all.

Additional context
Came up in microsoft/mcp#2618 and microsoft/fabric-rti-mcp#172. Workaround is setting "default": ""