[naga-cli] Use of --stdin-file-path prevents writing output files

[bavalpey]

Description
It's impossible to translate stdin files with naga-cli and specify the output file. It looks like the switch for parsing the --stdin-file-path argument is only invoked if the first file argument is Some: args.files.first(). However, this means that it's actually impossible to have naga translate a file from stdin.

Repro steps

$cat file.wgsl | naga --input-kind wgsl --stdin-file-path file.wgsl file.spv

Expected vs observed behavior
Using stdin-file-path should not cause naga to treat the first file name in the argument list as an input file.