Handle #to_spec #=> nil in RubyIndexer::Configuration (#1242)

Handle `#to_spec #=> nil`

If the dependency is a prerelease, `to_spec` may return `nil`. This
would crash ruby-lsp on boot.

Author: coffeejunk

lib/ruby_indexer/lib/ruby_indexer/configuration.rb

@@ -193,7 +193,7 @@ def initial_excluded_gems
       # When working on a gem, we need to make sure that its gemspec dependencies can't be excluded. This is necessary
       # because Bundler doesn't assign groups to gemspec dependencies
       this_gem = Bundler.definition.dependencies.find do |d|
-        d.to_spec.full_gem_path == Dir.pwd
+        d.to_spec&.full_gem_path == Dir.pwd
       rescue Gem::MissingSpecError
         false
       end