algo: Fix NPE if ClassType is not specified

algo: Fix NPE if ClassType is not specified

Path computation algorithms failed during edge pruning if ClassType
is not specified raising a NPE.

In addition, setting ClassType to 0 by default ito handle the NPE is
not conform to RFC5455.

This patch modify edge pruning when ClassType is not specified (set
to 0) and remove default setting to avoid problem when enforcing LSP
with a ClassType equal 0.

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>
Change-Id: I8930393359960dc9406fb9cc112bc194049cb35a
(cherry picked from commit 42282fe0eac1a45e0afdedd55c4984195ceddb8d)