Add nil matcher

src/spectator/matchers/nil_matcher.cr

@@ -0,0 +1,25 @@
+require "./value_matcher"
+
+module Spectator::Matchers
+  # Common matcher that tests whether a value is nil.
+  # The values are compared with the `#nil?` method.
+  struct NilMatcher < ConditionMatcher
+    # Determines whether the matcher is satisfied with the value given to it.
+    # True is returned if the match was successful, false otherwise.
+    def match?(partial : Expectations::ValueExpectationPartial(ActualType)) : Bool forall ActualType
+      partial.actual.nil?
+    end
+
+    # Describes the condition that satisfies the matcher.
+    # This is informational and displayed to the end-user.
+    def message(partial : Expectations::ValueExpectationPartial(ActualType)) : String forall ActualType
+      "Expected #{partial.label} to be nil"
+    end
+
+    # Describes the condition that won't satsify the matcher.
+    # This is informational and displayed to the end-user.
+    def negated_message(partial : Expectations::ValueExpectationPartial(ActualType)) : String forall ActualType
+      "Expected #{partial.label} to not be nil"
+    end
+  end
+end