[Bugfix] export for zero height

[yun-yeo]

Summary of changes

close #615

Report of required housekeeping

• Github issue OR spec proposal link
• Wrote tests
• Updated API documentation (client/lcd/swagger-ui/swagger.yaml)
• Added a relevant changelog entry: clog add [section] [stanza] [message]

---

(FOR ADMIN) Before merging

• Added appropriate labels to PR
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Confirm added tests are consistent with the intended behavior of changes
• Ensure all tests pass

[codecov]

Codecov Report

Merging #621 (cdffb46) into main (c080a55) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #621   +/-   ##
=======================================
  Coverage   44.74%   44.74%           
=======================================
  Files         120      120           
  Lines        6969     6968    -1     
=======================================
  Hits         3118     3118           
+ Misses       3613     3612    -1     
  Partials      238      238           

Impacted Files	Coverage Δ
app/export.go	0.00% <0.00%> (ø)