fix case where an environment variable containing an empty string is parsed into a slice or map

parse.go

@@ -441,13 +441,17 @@ func (p *Parser) captureEnvVars(specs []*spec, wasPresent map[*spec]bool) error
 		if spec.cardinality == multiple {
 			// expect a CSV string in an environment
 			// variable in the case of multiple values
-			values, err := csv.NewReader(strings.NewReader(value)).Read()
-			if err != nil {
-				return fmt.Errorf(
-					"error reading a CSV string from environment variable %s with multiple values: %v",
-					spec.env,
-					err,
-				)
+			var values []string
+			var err error
+			if len(strings.TrimSpace(value)) > 0 {
+				values, err = csv.NewReader(strings.NewReader(value)).Read()
+				if err != nil {
+					return fmt.Errorf(
+						"error reading a CSV string from environment variable %s with multiple values: %v",
+						spec.env,
+						err,
+					)
+				}
 			}
 			if err = setSliceOrMap(p.val(spec.dest), values, !spec.separate); err != nil {
 				return fmt.Errorf(

parse_test.go

@@ -721,6 +721,15 @@ func TestEnvironmentVariableSliceArgumentString(t *testing.T) {
 	assert.Equal(t, []string{"bar", "baz, qux"}, args.Foo)
 }
 
+func TestEnvironmentVariableSliceEmpty(t *testing.T) {
+	var args struct {
+		Foo []string `arg:"env"`
+	}
+	_, err := parseWithEnv("", []string{`FOO=`}, &args)
+	require.NoError(t, err)
+	assert.Len(t, args.Foo, 0)
+}
+
 func TestEnvironmentVariableSliceArgumentInteger(t *testing.T) {
 	var args struct {
 		Foo []int `arg:"env"`
@@ -775,6 +784,15 @@ func TestEnvironmentVariableMap(t *testing.T) {
 	assert.Equal(t, "ninetynine", args.Foo[99])
 }
 
+func TestEnvironmentVariableEmptyMap(t *testing.T) {
+	var args struct {
+		Foo map[int]string `arg:"env"`
+	}
+	_, err := parseWithEnv("", []string{`FOO=`}, &args)
+	require.NoError(t, err)
+	assert.Len(t, args.Foo, 0)
+}
+
 func TestEnvironmentVariableIgnored(t *testing.T) {
 	var args struct {
 		Foo string `arg:"env"`