Fix a regression in coerce_with when coercion returns nil (#2026)

Previously nil was handled as valid value for coercion for integer parameters.

With refactoring introduced in 1.3.0 integer parameter coerced with a custom method can't coerce to nil any more.

This change reverts back to previous behavior where coercion to nil was allowed.

Author: misdoro

CHANGELOG.md

@@ -10,6 +10,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#2026](https://github.com/ruby-grape/grape/pull/2026): Fix a regression in `coerce_with` when coercion returns `nil` - [@misdoro](https://github.com/misdoro).
 * [#2025](https://github.com/ruby-grape/grape/pull/2025): Fix Decimal type category - [@kdoya](https://github.com/kdoya).
 * [#2019](https://github.com/ruby-grape/grape/pull/2019): Avoid coercing parameter with multiple types to an empty Array - [@stanhu](https://github.com/stanhu).
 

lib/grape/validations/types/custom_type_coercer.rb

@@ -60,7 +60,7 @@ def call(val)
         end
 
         def coerced?(val)
-          @type_check.call val
+          val.nil? || @type_check.call(val)
         end
 
         private

spec/grape/validations/validators/coerce_spec.rb

@@ -525,6 +525,46 @@ def self.parsed?(value)
         expect(last_response.body).to eq('3')
       end
 
+      context 'Integer type and coerce_with potentially returning nil' do
+        before do
+          subject.params do
+            requires :int, type: Integer, coerce_with: (lambda do |val|
+              if val == '0'
+                nil
+              elsif val.match?(/^-?\d+$/)
+                val.to_i
+              else
+                val
+              end
+            end)
+          end
+          subject.get '/' do
+            params[:int].class.to_s
+          end
+        end
+
+        it 'accepts value that coerces to nil' do
+          get '/', int: '0'
+
+          expect(last_response.status).to eq(200)
+          expect(last_response.body).to eq('NilClass')
+        end
+
+        it 'coerces to Integer' do
+          get '/', int: '1'
+
+          expect(last_response.status).to eq(200)
+          expect(last_response.body).to eq('Integer')
+        end
+
+        it 'returns invalid value if coercion returns a wrong type' do
+          get '/', int: 'lol'
+
+          expect(last_response.status).to eq(400)
+          expect(last_response.body).to eq('int is invalid')
+        end
+      end
+
       it 'must be supplied with :type or :coerce' do
         expect do
           subject.params do