lxml text retrieval (find_txt) shouldn't fail if no text element available

[hellt]

Hi maintainers,
I propose to enhance the find_txt function so it won't log an error when it tries to get the text out of the element that has no text element present.

Consider the following example:

import lxml
from napalm.base.helpers import find_txt

s = """
<port>
    <port-id>1/1/c5/1</port-id>
    <oper-state>up</oper-state>
    <system-enabled-capabilities></system-enabled-capabilities>
</port>
"""

e = lxml.etree.fromstring(s)

print(
    "element with text value:", find_txt(e, "oper-state", "default_value",),
)

print(
    "element with no text value:",
    find_txt(e, "system-enabled-capabilities", "default_value",),
)

In the XML snippet above, the current find_txt behaviour will log an error if one tries to get the text out of the element with not text value (second print func):

# python run_orig_find_txt.py
element with text value: up
'NoneType' object has no attribute 'strip'
element with no text value: default_value

The proposed PR adds additional conditional branch to not try to get the text value from a None object. Which will result in a clean execution:

# python run_new_find_txt.py
element with text value: up
element with no text value: default_value

[coveralls]

Coverage remained the same at 0.0% when pulling af3dbee on hellt:find_txt into 3e676cb on napalm-automation:develop.

[mirceaulinic]

LGTM. Thanks @hellt!