fix(converter): 修复 nginx 配置解析空字符串和 upstream URL

- 解析器正确处理空引号字符串 ""，使用标记区分空字符串和特殊字符
- upstream server URL 自动添加 http:// 前缀以满足 lolly 配置验证
- proxy_set_header 正确处理空字符串值（如 Connection ""）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/converter/nginx/converter.go

@@ -167,7 +167,12 @@ func convertUpstreamServer(d *Directive) config.ProxyTarget {
 	target := config.ProxyTarget{}
 
 	if len(d.Args) > 0 {
-		target.URL = d.Args[0]
+		url := d.Args[0]
+		// Add http:// prefix if no scheme present (nginx upstream servers don't have schemes)
+		if !strings.HasPrefix(url, "http://") && !strings.HasPrefix(url, "https://") {
+			url = "http://" + url
+		}
+		target.URL = url
 	}
 
 	for _, arg := range d.Args[1:] {
@@ -619,7 +624,12 @@ func convertProxyDirectives(directives []Directive, proxy *config.ProxyConfig, u
 				if proxy.Headers.SetRequest == nil {
 					proxy.Headers.SetRequest = make(map[string]string)
 				}
-				proxy.Headers.SetRequest[d.Args[0]] = mapVariable(d.Args[1], result, d)
+				value := mapVariable(d.Args[1], result, d)
+				// Convert the empty string marker back to actual empty string
+				if value == `""` {
+					value = ""
+				}
+				proxy.Headers.SetRequest[d.Args[0]] = value
 			}
 		case "proxy_hide_header":
 			if len(d.Args) > 0 {

internal/converter/nginx/converter_test.go

@@ -159,8 +159,8 @@ http {
 		t.Fatalf("expected 3 targets, got %d", len(p.Targets))
 	}
 
-	if p.Targets[0].URL != "10.0.0.1:8080" {
-		t.Errorf("target[0] URL = %s, want 10.0.0.1:8080", p.Targets[0].URL)
+	if p.Targets[0].URL != "http://10.0.0.1:8080" {
+		t.Errorf("target[0] URL = %s, want http://10.0.0.1:8080", p.Targets[0].URL)
 	}
 	if p.Targets[0].Weight != 3 {
 		t.Errorf("target[0] Weight = %d, want 3", p.Targets[0].Weight)
@@ -1244,11 +1244,11 @@ http {
 	if len(p.Targets) != 2 {
 		t.Fatalf("expected 2 targets from upstream inside http block, got %d", len(p.Targets))
 	}
-	if p.Targets[0].URL != "10.0.0.1:8080" {
-		t.Errorf("target[0] URL = %s, want 10.0.0.1:8080", p.Targets[0].URL)
+	if p.Targets[0].URL != "http://10.0.0.1:8080" {
+		t.Errorf("target[0] URL = %s, want http://10.0.0.1:8080", p.Targets[0].URL)
 	}
-	if p.Targets[1].URL != "10.0.0.2:8080" {
-		t.Errorf("target[1] URL = %s, want 10.0.0.2:8080", p.Targets[1].URL)
+	if p.Targets[1].URL != "http://10.0.0.2:8080" {
+		t.Errorf("target[1] URL = %s, want http://10.0.0.2:8080", p.Targets[1].URL)
 	}
 }
 

internal/converter/nginx/parser.go

@@ -280,7 +280,16 @@ func (p *parser) readToken() (string, error) {
 	ch := p.input[p.pos]
 
 	if ch == '"' || ch == '\'' {
-		return p.readQuotedString(ch)
+		token, err := p.readQuotedString(ch)
+		if err != nil {
+			return "", err
+		}
+		// Return a special marker for empty quoted strings to distinguish
+		// from the empty string returned when encountering special chars.
+		if token == "" {
+			return `""`, nil
+		}
+		return token, nil
 	}
 
 	if ch == '{' || ch == '}' || ch == ';' {