Fix segfault with uninitialized JogArm thread

[henningkayser]

Starting the main loop in a thread and immediately calling JogCppAPI::getCommandFrameTransform() can result in a segfault if jog_calcs_ has not been created yet. This change checks if jog_calcs_ is not null before testing jog_calcs_->isInitialized() so that early calls to JogCppAPI::getCommandFrameTransform() return false.

[codecov-io]

Codecov Report

Merging #1882 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1882      +/-   ##
==========================================
- Coverage   50.28%   50.27%   -0.02%     
==========================================
  Files         313      313              
  Lines       24630    24621       -9     
==========================================
- Hits        12385    12377       -8     
+ Misses      12245    12244       -1

Impacted Files	Coverage Δ
...erimental/moveit_jog_arm/src/jog_cpp_interface.cpp	12.04% <0%> (+0.14%)	⬆️
...ccupancy_map_monitor/src/occupancy_map_updater.cpp	4.76% <0%> (-4.33%)	⬇️
...ccupancy_map_monitor/src/occupancy_map_monitor.cpp	29.65% <0%> (-0.96%)	⬇️
moveit_core/planning_scene/src/planning_scene.cpp	45.99% <0%> (-0.15%)	⬇️
...anning_scene_monitor/src/current_state_monitor.cpp	53.09% <0%> (+0.27%)	⬆️
...xperimental/moveit_jog_arm/src/low_pass_filter.cpp	100% <0%> (+4.54%)	⬆️

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[AndyZe]

Looks perfect!