initialize sensors list (#166)

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# PR Details
## Description

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Sensor list is initialized as an empty array so an exception is not
thrown if there is no sensor configuration.
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## Related Issue
[CDAR-427](https://usdot-carma.atlassian.net/browse/CDAR-427)
## Motivation and Context

If a sensor configuration file is not available, registration throws a
NullPointerException exception causing CDASim to crash. This is not the
desired behavior, since there may be times when a sensor config does not
exist.

 The solution is to initialize the sensors list as an empty list.
## How Has This Been Tested?
local integration tested
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## Types of changes

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- [x] Defect fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that cause existing functionality
to change)

## Checklist:

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- [ ] I have added any new packages to the sonar-scanner.properties file
- [ ] My change requires a change to the documentation.
- [ ] I have updated the documentation accordingly.
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[CARMA Contributing Guide](Contributing.md) 
- [ ] I have added tests to cover my changes.
- [x] All new and existing tests passed.

co-simulation/fed/mosaic-infrastructure/src/main/java/org/eclipse/mosaic/fed/infrastructure/ambassador/InfrastructureInstance.java

@@ -21,6 +21,8 @@
 import java.net.DatagramSocket;
 import java.net.InetAddress;
 import java.util.List;
+import java.util.ArrayList;
+
 
 import org.eclipse.mosaic.lib.geo.CartesianPoint;
 import org.eclipse.mosaic.lib.objects.detector.DetectedObject;
@@ -44,7 +46,7 @@ public class InfrastructureInstance {
     private int simulatedInteractionPort;
     private CartesianPoint location = null;
     private DatagramSocket socket = null;
-    private List<Detector> sensors;
+    private List<Detector> sensors = new ArrayList<>();
 
     /**
      * Constructor for InfrastructureInstance