Show empty state when not in any session

[nweiyue]

No description provided.

[codecov-io]

Codecov Report

Merging #230 into master will decrease coverage by 0.29%.
The diff coverage is 6.25%.

@@             Coverage Diff              @@
##             master     #230      +/-   ##
============================================
- Coverage     71.58%   71.28%   -0.30%     
  Complexity      964      964              
============================================
  Files           134      134              
  Lines          2953     2964      +11     
  Branches        307      308       +1     
============================================
- Hits           2114     2113       -1     
- Misses          739      750      +11     
- Partials        100      101       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/atas/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ain/java/atas/ui/sessionlist/SessionListPanel.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...i/sessionlist/session/SessionStudentListPanel.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ain/java/atas/ui/studentlist/StudentListPanel.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
src/main/java/atas/logic/LogicManager.java	83.33% <100.00%> (+0.40%)	12.00 <1.00> (+1.00)
.../java/atas/model/statistics/SessionStatistics.java	92.45% <0.00%> (-3.78%)	22.00% <0.00%> (-1.00%)

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[marcustw]

LGTM!