Pretty service inspect fails when TaskTemplate.Resources is nil

[akerouanton]

- What I did

When doing docker service inspect --pretty on services without
TaskTemplate.Resources or TaskTemplate.Resources.Limits, the command
fails. This is due to a missing check on ResourceLimitPids().

This bug has been introduced by 395a6d5
and produces following error message:

Template parsing error: template: :139:10: executing "" at <.ResourceLimitPids>: error calling ResourceLimitPids: runtime error: invalid memory address or nil pointer dereference

- How I did it

- How to verify it

$ make binary # Build the binary from master branch
$ cat docker-compose.yaml
version: "3"

services:
  test:
    image: debian
    command: sleep 15
    deploy:
      mode: replicated
$ docker stack deploy --compose-file docker-compose.yaml test
$ build/docker service inspect --pretty test_test

- Description for the changelog

docker service inspect --pretty fails on tasks declared with no resources constraints.

- A picture of a cute animal (not mandatory but encouraged)

[cpuguy83]

LGTM

[codecov-commenter]

Codecov Report

Merging #2661 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2661      +/-   ##
==========================================
- Coverage   58.14%   58.13%   -0.01%     
==========================================
  Files         295      295              
  Lines       21198    21200       +2     
==========================================
  Hits        12325    12325              
- Misses       7966     7967       +1     
- Partials      907      908       +1     

[thaJeztah]

This bug has been introduced by 395a6d5

Whoops! Thanks for fixing 😅

[thaJeztah]

LGTM