fix: correct example config to use null and add validation test (#209)

Author: yshngg

.testcoverage.example.yml

@@ -60,12 +60,12 @@ diff:
   # Allowed threshold for the test coverage difference (in percentage) 
   # between the feature branch and the base branch.
   #
-  # By default, this is disabled (set to nil). Valid values range from 
+  # By default, this is disabled (set to null). Valid values range from 
   # -100.0 to +100.0.
   #
   # Example: 
   #   If set to 0.5, an error will be reported if the feature branch has 
   #   less than 0.5% more coverage than the base.
   #
   #   If set to -0.5, the check allows up to 0.5% less coverage than the base.
-  threshold: nil
+  threshold: null

pkg/testcoverage/config_test.go

@@ -213,6 +213,15 @@ func Test_ConfigFromFile(t *testing.T) {
 		assert.NoError(t, err)
 		assert.Equal(t, savedCfg, cfg)
 	})
+
+	t.Run("example file", func(t *testing.T) {
+		t.Parallel()
+
+		cfg := Config{}
+		filename := "../../.testcoverage.example.yml"
+		err := ConfigFromFile(&cfg, filename)
+		assert.NoError(t, err)
+	})
 }
 
 func TestConfigYamlParse(t *testing.T) {