Don't throw exception on empty path list

[tzaeschke]

PathPolicies should accept or produce empty path lists without throwing an exception,

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/main/java/org/scion/jpan/PathPolicy.java	92.85%	0 Missing and 1 partial ⚠️
.../main/java/org/scion/jpan/ScionDatagramSocket.java	66.66%	1 Missing ⚠️

Files with missing lines	Coverage Δ	Complexity Δ
...n/java/org/scion/jpan/AbstractDatagramChannel.java	85.21% <100.00%> (+0.37%)	88.00 <2.00> (+2.00)
...main/java/org/scion/jpan/ScionDatagramChannel.java	82.60% <100.00%> (+0.15%)	24.00 <0.00> (ø)
src/main/java/org/scion/jpan/ppl/PplPolicy.java	82.96% <ø> (+0.94%)	37.00 <0.00> (-1.00)	⬆️
src/main/java/org/scion/jpan/PathPolicy.java	97.82% <92.85%> (-0.22%)	1.00 <0.00> (-2.00)
.../main/java/org/scion/jpan/ScionDatagramSocket.java	55.34% <66.66%> (ø)	59.00 <0.00> (ø)

... and 1 file with indirect coverage changes