MemoizedValue memoizes nil results (#2053)

I moved the existing `subject` into the test because it didn't make sense for the second test I added.

lib/pry/config/memoized_value.rb

@@ -19,11 +19,15 @@ class Pry
     class MemoizedValue
       def initialize(&block)
         @block = block
+        @called = false
         @call = nil
       end
 
       def call
-        @call ||= @block.call
+        return @call if @called
+
+        @called = true
+        @call = @block.call
       end
     end
   end

spec/config/memoized_value_spec.rb

@@ -2,10 +2,20 @@
 
 RSpec.describe Pry::Config::MemoizedValue do
   describe "#call" do
-    subject { described_class.new { rand } }
-
     it "memoizes the result of call" do
-      expect(subject.call).to eq(subject.call)
+      instance = described_class.new { rand }
+      expect(instance.call).to eq(instance.call)
+    end
+
+    it "doesn't conflate falsiness with unmemoizedness" do
+      count = 0
+      instance = described_class.new do
+        count += 1
+        nil
+      end
+      expect(instance.call).to eq nil
+      expect(instance.call).to eq nil
+      expect(count).to eq 1
     end
   end
 end