Fix syntax errors on string comparator

[atownsend247]

When running via ansible 2.10.7 you are presented with a few SyntaxWarnings everytime a function is referenced, see below code block, this PR resolves this by swapping the is comparator for empty strings to == instead.

12:24:46  TASK [Get the current caller identity information] *****************************
12:24:47  ok: [localhost]
12:24:47  
12:24:47  TASK [update kube config] ******************************************************
12:24:47  /opt/ansible/plugins/ansible-filter-plugins/plugins/filters/string_utils.py:51: SyntaxWarning: "is" with a literal. Did you mean "=="?
12:24:47    if needle is '' or needle is None:
12:24:47  /opt/ansible/plugins/ansible-filter-plugins/plugins/filters/string_utils.py:71: SyntaxWarning: "is" with a literal. Did you mean "=="?
12:24:47    if sanitzed_string is '' or sanitzed_string is None:
12:24:47  /opt/ansible/plugins/ansible-filter-plugins/plugins/filters/string_utils.py:81: SyntaxWarning: "is" with a literal. Did you mean "=="?
12:24:47    if sanitzed_string is '' or sanitzed_string is None:
12:24:47  /opt/ansible/plugins/ansible-filter-plugins/plugins/filters/string_utils.py:116: SyntaxWarning: "is" with a literal. Did you mean "=="?
12:24:47    if string is '' or string is None:
12:24:58  ok: [localhost]