Docs JSON output skips return type for methods without args

[oprypin]

For each method, crystal docs --format=json outputs the value args_html.
That value includes both the args and the return value, e.g.
for the method def some_method(name : String) : Int32 and its variations:

1. "args_html": "(name : String) : Int32"
2. "args_html": "(name : String)" (missing return value, OK)
3. "args_html": " : Int32" (method has no args, OK)

However, since #11438 (released in 1.3.0) having no args makes the args_html value totally omitted, so the 3rd case is broken and so https://github.com/mkdocstrings/crystal stopped outputting any return value in such case.

The problematic line:

crystal/src/compiler/crystal/tools/doc/method.cr

Line 330 in 35afca6

builder.field "args_html", args_to_html unless args.empty?

Thanks to @Blacksmoke16 who spotted this problem in https://athenaframework.org/ImageSize/#Athena::ImageSize.dpi

[Sija]

Given that field is suppose to return the arguments, including the return type seems like a mistake. Wouldn't be better to have additional field just for that?