fix(client): #1182 filter out empty extra arguments for fortls extraArgs

[folk85]

Empty variable in the ExtraArgs list are included into the global argument list and then fortls try to parse it. Here is the simple example which reproduce the problem:

#file:./.vscode/settings.json
    "fortran.fortls.extraArgs": [
        "--max_comment_line_length=500",
        "",
        "",
        "",
        "",
        "--max_line_length=500",
    ],

The Output panel of "Modern Fortran" shows the following error:

If we filter the argument list eliminating empty arguments the problem is dissappearing: