replace homePath with default if it's empty

[Vritra4]

if we run initiad without --home, L421 returns empty string.
so i think replacing with DefaultNodeHome if homePath is empty will prevent upgradeKeeper's odd behavior(downloading binaries into cwd i guess).

if this pr approved, i'll make a pr for cosmos-sdk/simapp also :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (44bed60) 38.87% compared to head (6373777) 38.89%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   38.87%   38.89%   +0.01%     
==========================================
  Files         162      162              
  Lines       16749    16752       +3     
==========================================
+ Hits         6512     6515       +3     
  Misses       9138     9138              
  Partials     1099     1099              

Files	Coverage Δ
app/app.go	85.06% <100.00%> (+0.05%)	⬆️