Fix split() to have same behavior for single-char FS

Fix case of empty input line (NF should be 0)

interp/functions.go

@@ -15,6 +15,7 @@ import (
 	"strings"
 	"syscall"
 	"time"
+	"unicode/utf8"
 
 	. "github.com/benhoyt/goawk/internal/ast"
 	. "github.com/benhoyt/goawk/lexer"
@@ -540,7 +541,11 @@ func (p *interp) split(s string, scope VarScope, index int, fs string) (int, err
 	var parts []string
 	if fs == " " {
 		parts = strings.Fields(s)
-	} else if s != "" {
+	} else if s == "" {
+		// NF should be 0 on empty line
+	} else if utf8.RuneCountInString(fs) <= 1 {
+		parts = strings.Split(s, fs)
+	} else {
 		re, err := p.compileRegex(fs)
 		if err != nil {
 			return 0, err

interp/io.go

@@ -291,11 +291,11 @@ func (p *interp) ensureFields() {
 	if p.fieldSep == " " {
 		// FS space (default) means split fields on any whitespace
 		p.fields = strings.Fields(p.line)
+	} else if p.line == "" {
+		p.fields = nil
 	} else if utf8.RuneCountInString(p.fieldSep) <= 1 {
 		// 1-char FS is handled as plain split (not regex)
 		p.fields = strings.Split(p.line, p.fieldSep)
-	} else if p.line == "" {
-		p.fields = nil
 	} else {
 		// Split on FS as a regex
 		p.fields = p.fieldSepRegex.Split(p.line, -1)