Fix missing path in case of solitary core AS

[tzaeschke]

The path construction algorithm failed to construct a path when it received an UP segment and an empty list of CORE segments.
The problem occurred because the algorithm was checking for list==null instead of list==empty.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/main/java/org/scion/jpan/internal/Segments.java	94.44%	0 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ	Complexity Δ
...rc/main/java/org/scion/jpan/internal/Segments.java	88.13% <94.44%> (+2.54%)	107.00 <15.00> (+11.00)

... and 2 files with indirect coverage changes

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