fix onUnknownRoute exception on back button in web

[limexp]

This PR fixes null-parameter exception in case of using onUnknownRoute.

pathTemplate could be null if onUnknownRoute is matched, but null is not accepted in fromRouteInformation https://github.com/tomgilder/routemaster/blob/main/lib/src/route_data.dart#L166

To generate an exception:

1. add onUnknownRoute handler (onUnknownRoute: (path) { return MaterialPage(child: ProfilePage()); }, for simple_example)
2. visit onUnknownRoute handled page and any next page
3. press back button in web-browser to return to onUnknownRoute handled page

The following TypeErrorImpl was thrown while dispatching notifications for PlatformRouteInformationProvider:
Expected a value of type 'String', but got one of type 'Null'
packages/routemaster/src/route_data.dart 166:45 fromRouteInformation

[codecov]

Codecov Report

Merging #300 (e6bc6d6) into main (620e070) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #300   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files          20       20           
  Lines        1005     1005           
=======================================
  Hits          999      999           
  Misses          6        6           

Files Changed	Coverage Δ
lib/src/route_data.dart	100.00% <100.00%> (ø)

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