Setting all periods to False is causing an error

[mlech-reef]

Describe the bug
Setting all periods to False is causing an error. See the traceback below.

SENTRY_ISSUES_1H=False
SENTRY_ISSUES_24H=False
SENTRY_ISSUES_14D=False

To Reproduce
Set:

SENTRY_ISSUES_1H=False
SENTRY_ISSUES_24H=False
SENTRY_ISSUES_14D=False

Expected behavior
If at least one is required, we should get a nice error message regarding misconfiguration.

Screenshots

maj 28 20:09:25 maciek-xps sentry-exporter[5355]: Traceback (most recent call last):                                                                                                                               
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2073, in wsgi_app                                                                             
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     response = self.full_dispatch_request()                                                                                                                      
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1518, in full_dispatch_request                                                                
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     rv = self.handle_user_exception(e)                                                                                                                           
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1516, in full_dispatch_request                                                                
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     rv = self.dispatch_request()                                                                                                                                 
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1502, in dispatch_request                                                                     
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)                                                                                 
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/usr/local/lib/python3.7/site-packages/flask_httpauth.py", line 172, in decorated                                                                        
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     return self.ensure_sync(f)(*args, **kwargs)                                                                                                                  
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/app/exporter.py", line 101, in sentry_exporter                                                                                                          
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     REGISTRY.register(SentryCollector(sentry, ORG_SLUG, get_metric_config(), PROJECTS_SLUG))                                                                     
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/usr/local/lib/python3.7/site-packages/prometheus_client/registry.py", line 26, in register                                                              
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     names = self._get_names(collector)                                                                                                                           
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/usr/local/lib/python3.7/site-packages/prometheus_client/registry.py", line 66, in _get_names                                                            
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     for metric in desc_func():                                                                                                                                   
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:   File "/app/helpers/prometheus.py", line 248, in collect                                                                                                        
maj 28 20:09:25 maciek-xps sentry-exporter[5355]:     project_issues_1h = project_issues.get(env).get("1h")                                                                                                        
maj 28 20:09:25 maciek-xps sentry-exporter[5355]: AttributeError: 'NoneType' object has no attribute 'get'

Desktop (please complete the following information):

• OS: ubuntu 22.04
• Browser: curl 7.81.0

Additional context
N/A