Robot's get_link_names throws exception for group with one link

**Describe the bug**
Robot's get_link_names throws exception for group with one link

**To Reproduce**
Steps to reproduce the behavior:
1. Load the ur5 demo robot.
2. Run the following code:

```
try:
# If testing with the released versions of compas_fab up to 1.0.2, this code will run
from compas_fab.robots.ur5 import Robot
robot = Robot()
except:
# if testing with the main branch (unreleased), this one will do instead
from compas_fab.robots import RobotLibrary
robot = RobotLibrary.ur5()

names = robot.get_link_names(group="endeffector")
print(names)

```

**Expected behavior**
`names` should equal `['ee_link']`

**Actual behavior**
Exception is thrown "Exception: No chain found between the specified element"