🐛 Fix corner case handling for output permutation in defer measurements optimization (cda-tum#587)

## Description

This PR fixes a small corner case in the defer measurements optimization
pass where a circuit with no measurements would loose its output
permutation and have it reset to an identity permutation.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
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---------

Signed-off-by: burgholzer <burgholzer@me.com>

Author: burgholzer

src/CircuitOptimizer.cpp

@@ -1,7 +1,10 @@
 #include "CircuitOptimizer.hpp"
 
+#include "operations/NonUnitaryOperation.hpp"
+
 #include <algorithm>
 #include <cassert>
+#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -927,6 +930,9 @@ void CircuitOptimizer::deferMeasurements(QuantumComputation& qc) {
     }
     ++it;
   }
+  if (qubitsToAddMeasurements.empty()) {
+    return;
+  }
   qc.outputPermutation.clear();
   for (const auto& [qubit, clbit] : qubitsToAddMeasurements) {
     qc.measure(qubit, clbit);