Remove path part concatenation for paths without parameters and removed append and concatenation of single characters

[paulomorgado]

Improves on #4579.
Ddresses #4587.

Generated code is now:

// Operation Path: "sum/{a}/{b}"
urlBuilder_.Append("sum/");
urlBuilder_.Append(System.Uri.EscapeDataString(ConvertToString(a, System.Globalization.CultureInfo.InvariantCulture)));
urlBuilder_.Append('/');
urlBuilder_.Append(System.Uri.EscapeDataString(ConvertToString(b, System.Globalization.CultureInfo.InvariantCulture)));

// Operation Path: "abs({a})"
urlBuilder_.Append("abs(");
urlBuilder_.Append(System.Uri.EscapeDataString(ConvertToString(a, System.Globalization.CultureInfo.InvariantCulture)));
urlBuilder_.Append(')');

Also replaces appending a single character string with appending a character and replaces string concatenation to be appending with appending of the two strings.

[cristipufu]

@paulomorgado @RicoSuter @lahma can we please fast track this fix?

[RicoSuter]

Thanks for the PR.