Missing column in bar chart derived from data.json where the x value is 'empty'

C3 version: 0.7.11
D3 version: 5.14.2
Browser: Chrome Version 94.0.4606.81 (Official Build) (x86_64)
OS: macOs Mojave Version 10.14.6

Observed: The chart seems to want to filter out a column but is always filtering out the last column.

Expected: Keep the column entirely, or filter out the 'empty' column specifically.

Code snippet:
```
var chart = c3.generate({
data: {
json: [
{name: 'www.site1.com', upload: 200, download: 200, total: 400},
{name: '', upload: 100, download: 300, total: 400},
{name: 'www.site3.com', upload: 300, download: 200, total: 500},
{name: 'www.site4.com', upload: 400, download: 100, total: 500}
],
type:'bar',
keys: {
x: 'name', // it's possible to specify 'x' when category axis
value: ['upload', 'download']
}
},
axis: {
x: {
type: 'category'
}
}
});
```

Start by running the provided c3.generate snippet with the listed C3, D3, browser, and OS versions, then trace the category-axis handling for the empty `name` value. Done means the empty category is retained or specifically excluded rather than causing the final column to disappear, with coverage for this example.

Written by the indexing model from the issue text.