fix: pass empty contexts rather than null to UniAwait

[jponge]

When no context is specified, Context.empty() must be used rather than null.

Fixes #1457

[codecov]

Codecov Report

Merging #1458 (303bdab) into main (5e31dc6) will decrease coverage by 0.32%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1458      +/-   ##
============================================
- Coverage     89.37%   89.06%   -0.32%     
+ Complexity     3364     3352      -12     
============================================
  Files           459      459              
  Lines         13402    13402              
  Branches       1641     1641              
============================================
- Hits          11978    11936      -42     
- Misses          795      824      +29     
- Partials        629      642      +13     

Files	Coverage Δ
...java/io/smallrye/mutiny/operators/AbstractUni.java	90.32% <100.00%> (ø)

... and 13 files with indirect coverage changes