JTC trajectory end time validation fix (backport #1090)

[mergify]

The time_from_start member for a JointTrajectoryPoint is monotonically increasing, not a delta time, so I think we can just use the last one in the vector; summing them up seems incorrect. I also moved the points empty check earlier because .back().time_from_start would seg fault with an empty points vector.

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This is an automatic backport of pull request #1090 done by Mergify.

[codecov]

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@@            Coverage Diff            @@
##             humble    #1140   +/-   ##
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  Coverage          ?   86.81%           
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  Files             ?       86           
  Lines             ?     7486           
  Branches          ?      619           
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  Hits              ?     6499           
  Misses            ?      753           
  Partials          ?      234           

Flag	Coverage Δ
unittests	86.81% <100.00%> (?)

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Files	Coverage Δ
...ory_controller/src/joint_trajectory_controller.cpp	83.76% <100.00%> (ø)
...ory_controller/test/test_trajectory_controller.cpp	99.58% <100.00%> (ø)