[typescript-angular] improve small template issues

Created by: eseliger

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Fixes several minor issues in the templates for typescript-angular

• canConsumeForm is only included in the service if the method is actually needed. Reduces code size and helps with compiling the project where no unused locals cause compile errors
• no summary or no notes for an operation don't cause the jsdoc to start with an empty line
• only render consumes property when it is actually used. Also helpful when compiling the project with no unused locals
• fix formatting of doc block for encoder.ts

Anyone knows of a better solution to fix the consumes rather than having this "or" which needs the code to be duplicated? I didn't want to add another property to the CodegenOperation since it's only used for 1 generator. Maybe this is also acceptable since it'll be refactored soon anyways. Please advise :)

@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @nicokoenig (2018/09) @topce (2018/10) @akehir (2019/07)