convert : fix F32 ftype not being saved

[cebtenzzre]

params.ftype is falsy if it is GGMLFileType.AllF32. Use is not None instead of implicit boolean conversion.

before:

$ ./convert.py openlm-research_open_llama_3b --outfile openllama-3b.f32.gguf --outtype f32
$ ./main -m openllama-3b.f32.gguf
<snip>
llm_load_print_meta: model ftype    = all F32 (guessed)

after:

$ ./convert.py openlm-research_open_llama_3b --outfile openllama-3b.f32.gguf --outtype f32
$ ./main -m openllama-3b.f32.gguf
<snip>
llm_load_print_meta: model ftype    = all F32

[KerfuffleV2]

Possibly apply the same treatment for the other params nearby? Unless you specifically don't want to allow a rope scale of 0, etc — but in that case I think doing something other than just not saving it would probably be better.

edit: Looks good! I hate that truthy stuff.