Prevent spurious warning in views_taxonomy.inc

This is a trivial patch against HEAD that fixes a spurious warning in views_taxonomy.inc, resulting from trying to implode() an empty array:

warning: implode() [function.implode]: Bad arguments. in modules/views/modules/views_taxonomy.inc on line 257.
warning: implode() [function.implode]: Bad arguments. in modules/views/modules/views_taxonomy.inc on line 257.
warning: implode() [function.implode]: Bad arguments. in modules/views/modules/views_taxonomy.inc on line 257.
...

The warning is generated (with 5.x-1.3-beta1, but HEAD has the same issue) when showing a table view that has taxonomy fields, and not all nodes have terms selected in those taxonomy fields.