fix: correct loose config string check

[jdlms]

What kind of change does this PR introduce?

[x ] Bugfix
[] Feature
[] Code update (local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

If the user has no config file, and has only set their api key as an environment variable, Anthropic is always being set as the default model. If you print out the viper config after setting OPENAI_API_KEY you will see this:

 {
  "$schema": "./opencode-schema.json",
  "agents": {
    "coder": {
      "model": "claude-3.7-sonnet"
    },
    "task": {
      "model": "claude-3.7-sonnet"
    },
    "title": {
      "model": "claude-3.7-sonnet"
    }
  },

If using an OpenAI api key, this results in the following error because the max_token size is defaulting to Claude's (see #140):

│Message:                                                                                                                                  │
│  POST "https://api.openai.com/v1/chat/completions": 400 Bad Request {                                                                    │
│      "message": "max_tokens is too large: 50000. This model supports at most 32768 completion tokens, whereas you provided 50000.",      │
│      "type": "invalid_request_error",                                                                                                    │
│      "param": "max_tokens",                                                                                                              │
│      "code": "invalid_value"                                                                                                             │
│    }                                                                                                                                     │

It is happening because in config.go, viper.Get(...) returns an interface{}, and comparing that to "" is not reliable.

What is the new behavior?

Using viper.GetString, models are now correctly set based on environment variable api keys and max_tokens is no longer set to 50000 by default.

[rekram1-node]

Thank you @jdlms!