Fix using --stdout with jsondoc (#22925)

Fixes the assertion defect that happens when using `jsondoc --stdout`
(There is no outfile since its just stdout)

```
Error: unhandled exception: options.nim(732, 3) `not conf.outFile.isEmpty`  [AssertionDefect]
```

Also makes the output easier to parse by ending each module output with
a new line.

compiler/docgen.nim

@@ -1806,7 +1806,7 @@ proc writeOutputJson*(d: PDoc, useWarning = false) =
     "moduleDescription": modDesc,
     "entries": d.jEntriesFinal}
   if optStdout in d.conf.globalOptions:
-    write(stdout, $content)
+    writeLine(stdout, $content)
   else:
     let dir = d.destFile.splitFile.dir
     createDir(dir)

compiler/msgs.nim

@@ -725,6 +725,8 @@ proc genSuccessX*(conf: ConfigRef) =
   elif conf.outFile.isEmpty and conf.cmd notin {cmdJsonscript} + cmdDocLike + cmdBackends:
     # for some cmd we expect a valid absOutFile
     output = "unknownOutput"
+  elif optStdout in conf.globalOptions:
+    output = "stdout"
   else:
     output = $conf.absOutFile
   if conf.filenameOption != foAbs: output = output.AbsoluteFile.extractFilename