fix(templates): prevent nil panic

[wass3rw3rk]

when referencing a template that defines a global environment, but the base pipeline doesn't, pipeline.Environment will be nil as its zero value. when the template is parsed and attempting to set the global environment we currently cause a assignment to entry in nil map error. this fix initializes an empty environment.

test case:

.vela.yml:

version: "1"

metadata:
  render_inline: true

templates:
  - name: nil-panic
    source: .nil-panic.yml
    format: go
    type: file
    vars:
      foo: bar

.nil-panic.yml:

metadata:
  template: true

environment:
  foo: "bar"

stages:
  vendor:
    steps:
      - name: test
        image: alpine
        commands:
          - echo hello

workaround is to define environment: {} at the top level in the base pipeline .vela.yml

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.05%. Comparing base (2960513) to head (ab73c2b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1194   +/-   ##
=======================================
  Coverage   53.04%   53.05%           
=======================================
  Files         559      559           
  Lines       19743    19747    +4     
=======================================
+ Hits        10473    10477    +4     
  Misses       8701     8701           
  Partials      569      569           

Files with missing lines	Coverage Δ
compiler/native/parse.go	87.09% <100.00%> (+0.57%)	⬆️