Fix NoMethodError when a caller location has no path

`Thread::Backtrace::Location#path` returns nil for the Enumerator frames
(e.g. "Enumerator::Generator#each") that appear in `caller_locations` during
an external iteration of Enumerator (e.g. `Enumerator#next`).
When a `TracePoint` callback fires inside such an iteration,
`PowerAssert.internal_file?` raises `NoMethodError` and the callback reports:

```
power_assert: [BUG] Failed to trace: NoMethodError: undefined method 'start_with?' for nil
```

This happens in practice when a block-style assertion drives Capybara,
whose `Capybara::Result` iterates matched elements with `Enumerator#next`.

While #31 originally suspected a CRuby issue,
the type signature of `Thread::Backtrace::Location#path` in ruby/rbs allows nil
(`() -> String?`), and this nil case is observable at least on Ruby 2.6.10, 2.7.8, 3.1.5,
3.3.10, 3.4.10, and 4.0.6, so it seems reasonable for power_assert to tolerate nil either way.

Since `internal_file?` answers whether the given file belongs to the power_assert
library, treat locations without a path as non-internal ones.
Also guard `app_context?`, which walks caller locations the same way.

Closes #31.

Author: koic

lib/power_assert.rb

@@ -37,13 +37,15 @@ def app_caller_locations
     end
 
     def app_context?
-      top_frame = caller_locations.drop_while {|i| i.path.start_with?(POWER_ASSERT_LIB_DIR) }.first
+      top_frame = caller_locations.drop_while {|i| i.path&.start_with?(POWER_ASSERT_LIB_DIR) }.first
       top_frame and ! internal_file?(top_frame.path)
     end
 
     private
 
     def internal_file?(file)
+      return false unless file
+
       INTERNAL_LIB_DIRS.find do |_, dir|
         file.start_with?(dir)
       end

test/nil_path_location_test.rb

@@ -0,0 +1,21 @@
+require_relative 'test_helper'
+
+class TestNilPathLocation < Test::Unit::TestCase
+  include PowerAssertTestHelper
+
+  t do
+    obj = []
+    def obj.foo; self; end
+    enum = Enumerator.new {|y| y << obj.foo }
+    def enum.inspect; '#<Enumerator>'; end
+    assert_equal <<END.chomp, assertion_message {
+      enum.next.foo
+      |    |    |
+      |    |    []
+      |    []
+      #<Enumerator>
+END
+      enum.next.foo
+    }
+  end
+end