Bug: packages without PyPI Classifiers can't be properly parsed

[RaczeQ]

Bug

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• I have filled out the system info
• I have described the bug, filled in the expected outcome and the actual
  outcome including screenshots where appropriate

System info

• OS: Ubuntu
• Version: 20.04

Describe the bug

Trying to analyse a package that doesn't have any Classifiers on a PyPI results in an error.
Example packages:
https://pypi.org/project/kaleido/
https://pypi.org/project/jsbeautifier/

Error:

Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File ".../.venv/lib/python3.8/site-packages/licensecheck/__main__.py", line 6, in <module>
    cli()
  File ".../.venv/lib/python3.8/site-packages/licensecheck/__init__.py", line 90, in cli
    depsWithLicenses = get_deps.getDepsWithLicenses(
  File ".../.venv/lib/python3.8/site-packages/licensecheck/get_deps.py", line 116, in getDepsWithLicenses
    packages = packageinfo.getPackages(reqs)
  File ".../.venv/lib/python3.8/site-packages/licensecheck/packageinfo.py", line 122, in getPackages
    packageinfo.add(getPackageInfoLocal(requirement))
  File ".../.venv/lib/python3.8/site-packages/licensecheck/packageinfo.py", line 34, in getPackageInfoLocal
    lice = licenseFromClassifierlist(pkgMetadata.get_all("Classifier"))
  File ".../.venv/lib/python3.8/site-packages/licensecheck/packageinfo.py", line 102, in licenseFromClassifierlist
    for val in classifiers:
TypeError: 'NoneType' object is not iterable

Expected outcome

When the package has no classifiers, return UNKNOWN license.

Actual outcome

When the package has no classifiers, the error is thrown since the code expects classifierList to be a list.

[RaczeQ]

Closing this one, since #30 has been merged.