scancode crash when supplying PNG file to --license-policy

### Description

Sanity checks for `--license-policy` are missing leading to a crash.

### How To Reproduce

```
$ ./scancode -l scancode --spdx-tv /tmp/scancode.spdx --license-policy /tmp/tmp8ulw9skr.png
Setup plugins...
Collect file inventory...
Scan files for: licenses with 1 process(es)...
[####################] 2
ERROR: failed to run post-scan plugin: license-policy:
Traceback (most recent call last):
File "/home/armijn/git/scancode-toolkit/src/scancode/cli.py", line 1084, in run_codebase_plugins
plugin.process_codebase(codebase, **kwargs)
File "/home/armijn/git/scancode-toolkit/src/licensedcode/plugin_license_policy.py", line 77, in process_codebase
if has_policy_duplicates(license_policy):
File "/home/armijn/git/scancode-toolkit/src/licensedcode/plugin_license_policy.py", line 114, in has_policy_duplicates
policies = load_license_policy(license_policy_location).get('license_policies', [])
File "/home/armijn/git/scancode-toolkit/src/licensedcode/plugin_license_policy.py", line 141, in load_license_policy
conf_content = conf.read()
File "/usr/lib64/python3.10/codecs.py", line 322, in decode
(result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte

WARNING: Files are missing a SHA1 attribute. Incomplete SPDX document created.
Scanning done.
Some files failed to scan properly:
ERROR: failed to run post-scan plugin: license-policy:
Traceback (most recent call last):
File "/home/armijn/git/scancode-toolkit/src/scancode/cli.py", line 1084, in run_codebase_plugins
plugin.process_codebase(codebase, **kwargs)
File "/home/armijn/git/scancode-toolkit/src/licensedcode/plugin_license_policy.py", line 77, in process_codebase
if has_policy_duplicates(license_policy):
File "/home/armijn/git/scancode-toolkit/src/licensedcode/plugin_license_policy.py", line 114, in has_policy_duplicates
policies = load_license_policy(license_policy_location).get('license_policies', [])
File "/home/armijn/git/scancode-toolkit/src/licensedcode/plugin_license_policy.py", line 141, in load_license_policy
conf_content = conf.read()
File "/usr/lib64/python3.10/codecs.py", line 322, in decode
(result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte
Summary: licenses with 1 process(es)
Errors count: 1
Scan Speed: 1.83 files/sec.
Initial counts: 1 resource(s): 1 file(s) and 0 directorie(s)
Final counts: 1 resource(s): 1 file(s) and 0 directorie(s)
Timings:
scan_start: 2023-11-17T150048.155768
scan_end: 2023-11-17T150051.949987
setup_scan:licenses: 3.24s
setup: 3.24s
scan: 0.55s
total: 3.85s
Removing temporary files...done.
```

### System configuration

> For bug reports, it really helps us to know:

* What OS are you running on? (Windows/MacOS/Linux)
* What version of scancode-toolkit was used to generate the scan file?
* What installation method was used to install/run scancode? (pip/source download/other)