MapLabelValue is incorrect when mapping strings

**Describe the bug**
the issue is mainly with the string mapping

**To Reproduce**
```py
from monai.transforms import MapLabelValue
xform = MapLabelValue(["label3", "label2", "label1"], ["label3", "label2", "label1"])
xform([["label3", "label1"], ["label1", "label2"]])
```
output:
```
array([[0., 0.],
       [0., 0.]], dtype=float32)
```