Avoid max_age empty parameter on authorize_url

[campisanoentaksi]

I have an issue with the version 4.0.9 that includes the enhancement discussed in #95
There is no problem with the 4.0.8.

I'm using Keycloak, and I'm testing with the Keycloak provider as well as with the Custom one.

In both cases, the plugin probably reads params[:reauth] == 0, sets the max_age to nil, which causes an invalid request to Keycloak. In the request URL, I can see a max_age parameter without a value:
...code_challenge_method=S256&max_age&scope=...

and Keycloak tries to convert max_age to an int, causing an internal error, preventing the login:

Invalid request: java.lang.NumberFormatException: For input string: ""
	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:xx)
	at java.base/java.lang.Integer.parseInt(Integer.java:xxx)
	at java.base/java.lang.Integer.valueOf(Integer.java:xxx)
	at org.keycloak.protocol.oidc.endpoints.request.AuthzEndpointQueryStringParser.getIntParameter(AuthzEndpointQueryStringParser.java:xx)

In the pull request, I'm just compacting the hash used as input for the authorize_url function to generally avoid sending null keys.

[picman]

Excellent, thank you.