Fix incorrect return type in Walker.normalize_ast

As of Solargraph 0.51.0, I started hitting exceptions from walker.rb like this:

```
[WARN] wrong number of arguments (given 3, expected 1..2)
.../solargraph-rails/lib/solargraph/rails/walker.rb:78:in `initialize'
/Users/broz/src/solargraph-rails/lib/solargraph/rails/walker.rb:67:in `new'
/Users/broz/src/solargraph-rails/lib/solargraph/rails/walker.rb:67:in `from_source'
/Users/broz/src/solargraph-rails/lib/solargraph/rails/model.rb:15:in `process'
/Users/broz/src/solargraph-rails/lib/solargraph-rails.rb:48:in `block in local'
/Users/broz/src/solargraph-rails/lib/solargraph-rails.rb:61:in `run_feature'
/Users/broz/src/solargraph-rails/lib/solargraph-rails.rb:48:in `local'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/solargraph-0.51.0/lib/solargraph/convention.rb:28:in `block in for_local'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/3.1.0/set.rb:511:in `each_key'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/3.1.0/set.rb:511:in `each'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/solargraph-0.51.0/lib/solargraph/convention.rb:27:in `for_local'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/solargraph-0.51.0/lib/solargraph/source_map.rb:32:in `initialize'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/solargraph-0.51.0/lib/solargraph/source_map.rb:158:in `new'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/solargraph-0.51.0/lib/solargraph/source_map.rb:158:in `map'
/Users/broz/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/solargraph-0.51.0/lib/solargraph/api_map.rb:54:in `map'
/Users/broz/src/solargraph-rails/spec/helpers.rb:4:in `load_string'
/Users/broz/src/solargraph-rails/spec/solargraph-rails/model_spec.rb:80:in `block (3 levels) in <top (required)>'
```

It looks like it happens in cases where Solargraph's source.node is a ::Parser::AST::Node.  There's a special case in Walker.normalize_ast() that is triggering that does not look correct on its face - it should always return an array of nodes, since the result is splatted in Walker.from_source.

The spec added reproduces this in newer solargraphs.

lib/solargraph/rails/walker.rb

@@ -54,7 +54,7 @@ def self.normalize_ast(source)
         ast = source.node
 
         if ast.is_a?(::Parser::AST::Node)
-          ast
+          [ast]
         else
           NodeParser.parse_with_comments(source.code, source.filename)
         end

spec/solargraph-rails/model_spec.rb

@@ -75,6 +75,17 @@ class Transaction < ActiveRecord::Base
     assert_class_method(api_map, 'Transaction.positive', ['Class<Transaction>'])
   end
 
+  it 'handles primary_abstract_class without breaking' do
+    expect do
+      load_string 'app/models/application_record.rb',
+                  <<-RUBY
+                  class ApplicationRecord < ActiveRecord::Base
+                    primary_abstract_class
+                  end
+                  RUBY
+    end.not_to raise_error
+  end
+
   it 'generates scope methods with parameters' do
     load_string 'app/models/person.rb',
                 <<-RUBY