Fix error on block pattern and method call without receiver

[pocke]

A block pattern raises an error on a method call without the receiver.

For example:

$ cat test.rb
foo

$ querly find a{}.foo
Error: undefined method `type' for nil:NilClass
pattern: a{}.foo
Backtrace:
  /path/to/querly-1.3.0/lib/querly/pattern/expr.rb:174:in `test_node'
  /path/to/querly-1.3.0/lib/querly/pattern/expr.rb:195:in `test_receiver'
  /path/to/querly-1.3.0/lib/querly/pattern/expr.rb:182:in `test_node'
  /path/to/querly-1.3.0/lib/querly/pattern/expr.rb:159:in `=~'
  /path/to/querly-1.3.0/lib/querly/analyzer.rb:42:in `test_pair'
  /path/to/querly-1.3.0/lib/querly/analyzer.rb:34:in `block (2 levels) in find'
  /path/to/querly-1.3.0/lib/querly/node_pair.rb:25:in `each_subpair'
  /path/to/querly-1.3.0/lib/querly/analyzer.rb:33:in `block in find'
  /path/to/querly-1.3.0/lib/querly/analyzer.rb:32:in `each'
  /path/to/querly-1.3.0/lib/querly/analyzer.rb:32:in `find'
  /path/to/querly-1.3.0/lib/querly/cli/find.rb:23:in `start'
  /path/to/querly-1.3.0/lib/querly/cli.rb:104:in `find'
  /path/to/thor-1.2.1/lib/thor/command.rb:27:in `run'
  /path/to/thor-1.2.1/lib/thor/invocation.rb:127:in `invoke_command'
  /path/to/thor-1.2.1/lib/thor.rb:392:in `dispatch'
  /path/to/thor-1.2.1/lib/thor/base.rb:485:in `start'
  /path/to/querly-1.3.0/exe/querly:8:in `<top (required)>'
  /path/to/bin/querly:25:in `load'
  /path/to/bin/querly:25:in `<main>'

This patch fixes this problem