Api4 - Don't crash when displaying an empty table

[totten]

The -T flag requests that results be displayed as a table. Consider these four commands:

1. cv api4 Route.get +w 'name like %admin%' -T +s path
2. cv api4 Route.get +w 'name like %admin%' -T
3. cv api4 Route.get +w 'name like %zzz%' -T +s path
4. cv api4 Route.get +w 'name like %zzz%' -T

How do they behave?

1. Shows a table (with path header) and exits normally
2. Shows a table (with many column headers) and exits normally
3. Shows an empty table (with path header) and exits normally
4. Crashes because it cannot pick a header. (At least, on php81 it does.) (array_keys(): Argument #1 ($array) must be of type array, null given)

This patch changes (4) to behave more like (3) -- it will show an empty table and exit normally.

[colemanw]

Looks good to me.

[totten]

civibot, test this please