Fix issue 2601 (dont fail on empty /nix folder) (#2605)

## Summary

Fix for #2601

## How was it tested?

1. created empty /nix folder
2. devbox succeeds now installing nix

## Community Contribution License

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[Apache 2 License](https://www.apache.org/licenses/LICENSE-2.0).

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license the
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Author: ascknx

internal/fileutil/fileutil.go

@@ -51,6 +51,14 @@ func Exists(path string) bool {
 	return err == nil
 }
 
+func IsDirEmpty(path string) bool {
+	entries, err := os.ReadDir(path)
+	if err != nil {
+		return false
+	}
+	return len(entries) == 0
+}
+
 // FileContains checks if a given file at 'path' contains the 'substring'
 func FileContains(path, substring string) (bool, error) {
 	data, err := os.ReadFile(path)

internal/nix/install.go

@@ -25,8 +25,9 @@ func BinaryInstalled() bool {
 	return cmdutil.Exists("nix")
 }
 
-func dirExists() bool {
-	return fileutil.Exists("/nix")
+func dirExistsAndIsNotEmpty() bool {
+	dir := "/nix"
+	return fileutil.Exists(dir) && !fileutil.IsDirEmpty(dir)
 }
 
 var ensured = false
@@ -57,7 +58,7 @@ func EnsureNixInstalled(ctx context.Context, writer io.Writer, withDaemonFunc fu
 	if BinaryInstalled() {
 		return nil
 	}
-	if dirExists() {
+	if dirExistsAndIsNotEmpty() {
 		if _, err = SourceProfile(); err != nil {
 			return err
 		} else if BinaryInstalled() {