fix: implement nil check for CEL program in matchRoutingRule

[dmutterSF]

Avoid nil pointer dereference when evaluating expression.
Resolves #6036

Haven't written go before so this might not be the proper or most optimal solution

[yaron2]

program is an unexported (private) field of Expr, so the place to do the check is here:

dapr/pkg/expr/expr.go

Line 65 in 1ae5816

out, _, err := e.program.Eval(variables)

.

[codecov]

Codecov Report

Merging #6280 (ac0bae5) into master (3b0defb) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6280      +/-   ##
==========================================
- Coverage   64.40%   64.39%   -0.01%     
==========================================
  Files         189      189              
  Lines       18881    18885       +4     
==========================================
+ Hits        12160    12161       +1     
- Misses       5753     5759       +6     
+ Partials      968      965       -3     

Impacted Files	Coverage Δ
pkg/expr/expr.go	81.39% <100.00%> (+7.03%)	⬆️

... and 2 files with indirect coverage changes

[yaron2]

Thanks for your contribution @dmutterSF