Fix browser empty string flag parsing

Currently, passing a flag that doesn't contain a value to
K6_BROWSER_ARGS results in an argument with an equal sign suffix:

K6_BROWSER_ARGS=disable-site-isolation-trials

Results in:
disable-site-isolation-trials=

Instead of:
disable-site-isolation-trials

This PR fixes this problem.

chromium/browser_type.go

@@ -304,7 +304,15 @@ func parseArgs(flags map[string]any) ([]string, error) {
 	for name, value := range flags {
 		switch value := value.(type) {
 		case string:
-			args = append(args, fmt.Sprintf("--%s=%s", name, value))
+			var arg string
+			if strings.TrimSpace(value) != "" {
+				arg = fmt.Sprintf("--%s=%s", name, value)
+			} else {
+				// If the value is empty, we don't include it in the args list.
+				// Otherwise, it will produce "--name=" which is invalid.
+				arg = fmt.Sprintf("--%s", name)
+			}
+			args = append(args, arg)
 		case bool:
 			if value {
 				args = append(args, fmt.Sprintf("--%s", name))