handle missing provenance for non-evaluated result

If resultProxy has been created but the result has not been
evaluated then it shouldn't show missing provenance error
for that result.

This patch works together with previous one that fixes
error reporting on provenance creation.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

solver/llbsolver/solver.go

@@ -749,9 +749,6 @@ func getRefProvenance(ref solver.ResultProxy, br *provenanceBridge) (*provenance
 	}
 	p := ref.Provenance()
 	if p == nil {
-		if br.req != nil {
-			return nil, errors.Errorf("missing provenance for %s", ref.ID())
-		}
 		return nil, nil
 	}