fix(compiler): add instance only when metadata is not nil

[ecrupper]

Giving metadata default empty values when nil causes this environment section to set values it shouldn't set, resulting in the worker not supplying its own defaults. This causes local exec to not properly mount the workspace.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.79%. Comparing base (79667fd) to head (2e5f9ef).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1201   +/-   ##
=======================================
  Coverage   53.78%   53.79%           
=======================================
  Files         561      561           
  Lines       19950    19949    -1     
=======================================
+ Hits        10730    10731    +1     
+ Misses       8646     8645    -1     
+ Partials      574      573    -1     

Files with missing lines	Coverage Δ
compiler/native/compile.go	71.83% <100.00%> (+0.60%)	⬆️