api: make exp_show return float instead of None for zeroes

[dberenbaum]

Handles zeroes in dvc.api.exp_show(). See the bar column for the tawie-purl below.

Before this PR:

>>> import pandas as pd
>>> from dvc.api import exp_show
>>> pd.DataFrame(exp_show())
   Experiment        rev          typ   Created parent State Executor  foo   bar
0        None  workspace     baseline      None   None  None     None  NaN  None
1        None       main     baseline  12:16 PM   None  None     None  NaN  None
2  tawie-purl    6b6cf80  branch_base  02:40 PM   None  None     None  1.0  None

After this PR:

>>> import pandas as pd
>>> from dvc.api import exp_show
>>> pd.DataFrame(exp_show())
   Experiment        rev          typ   Created parent State Executor  foo  bar
0        None  workspace     baseline      None   None  None     None  NaN  NaN
1        None       main     baseline  12:16 PM   None  None     None  NaN  NaN
2  tawie-purl    6b6cf80  branch_base  02:40 PM   None  None     None  1.0  0.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
dvc/api/experiments.py	100.00% <100.00%> (+8.33%)	⬆️
tests/func/api/test_experiments.py	100.00% <100.00%> (ø)

... and 48 files with indirect coverage changes

📢 Thoughts on this report? Let us know!.