Emitter crash if no tspconfig file

[lmazuel]

tspconfig is an optional file, it shouldn't be necessary.

The following steps crash the emitter:

tsp init  # Chose generic restapi
tsp install 
npm install @typespec/http-client-python
tsp compile . --emit @typespec/http-client-python

Error log:

Emitter "@typespec/http-client-python" crashed! This is a bug.
Please file an issue at https://github.com/microsoft/typespec/issues

TypeError: Cannot read properties of undefined (reading 'toLowerCase')
    at addDefaultOptions (file:///tmp/testtsp/node_modules/@typespec/http-client-python/dist/emitter/emitter.js:36:102)
    at Object.$onEmit [as emitFunction] (file:///tmp/testtsp/node_modules/@typespec/http-client-python/dist/emitter/emitter.js:60:5)
    at async runEmitter (file:///tmp/testtsp/node_modules/@typespec/compiler/dist/src/core/program.js:366:13)
    at async compile (file:///tmp/testtsp/node_modules/@typespec/compiler/dist/src/core/program.js:123:9)
    at async compileOnce (file:///tmp/testtsp/node_modules/@typespec/compiler/dist/src/core/cli/actions/compile/compile.js:36:25)
    at async compileAction (file:///tmp/testtsp/node_modules/@typespec/compiler/dist/src/core/cli/actions/compile/compile.js:19:9)

[msyyc]

The root cause is for empty main.tsp, python emitter doesn't check whether there is valid namespace defined then crash.