Support empty string OpenAPI identifiers (#59)

### Motivation

The OpenAPI specification allows the use of the empty string in some
places, for example in the following enum, which we've seen in some
real-world OpenAPI documents:

```yaml
type: string
enum:
  - ""
  - foo
  - bar
```

We have code that converts an OpenAPI identifier to a suitable string
that can be used as a Swift identifier. It currently returns an empty
string when given an empty string as input, which is not a valid Swift
identifier. We already have some logic there to avoid clashing with some
Swift keywords, so this can be extended to handle the empty string.

### Modifications

- Map the empty string OpenAPI identifier to "_empty"

### Result

We can generate compiling code when presented with empty string OpenAPI
identifiers.

### Test Plan

- Added unit test.
- Extended reference test.

### Resolves

- Resolves #33.
- Fixes #61.

---------

Signed-off-by: Si Beaumont <[email protected]>

Author: simonjbeaumont

Sources/_OpenAPIGeneratorCore/Extensions/String.swift

@@ -63,7 +63,7 @@ fileprivate extension String {
     /// ensures that the identifier starts with a letter and not a number.
     var sanitizedForSwiftCode: String {
         guard !isEmpty else {
-            return self
+            return "_empty"
         }
 
         // Only allow [a-zA-Z][a-zA-Z0-9_]*

Tests/OpenAPIGeneratorCoreTests/Extensions/Test_String.swift

@@ -29,6 +29,9 @@ final class Test_String: Test_Core {
 
             // Reserved name
             ("Type", "_Type"),
+
+            // Empty string
+            ("", "_empty"),
         ]
         for (input, sanitized) in cases {
             XCTAssertEqual(input.asSwiftSafeName, sanitized)

Tests/OpenAPIGeneratorReferenceTests/Resources/Docs/petstore.yaml

@@ -35,6 +35,7 @@ paths:
             - water
             - land
             - air
+            - ""
             type: string
         - name: feeds
           in: query

Tests/OpenAPIGeneratorReferenceTests/Resources/ReferenceSources/Petstore/Types.swift

@@ -691,13 +691,15 @@ public enum Operations {
                     case water
                     case land
                     case air
+                    case _empty
                     /// Parsed a raw value that was not defined in the OpenAPI document.
                     case undocumented(String)
                     public init?(rawValue: String) {
                         switch rawValue {
                         case "water": self = .water
                         case "land": self = .land
                         case "air": self = .air
+                        case "": self = ._empty
                         default: self = .undocumented(rawValue)
                         }
                     }
@@ -707,9 +709,10 @@ public enum Operations {
                         case .water: return "water"
                         case .land: return "land"
                         case .air: return "air"
+                        case ._empty: return ""
                         }
                     }
-                    public static var allCases: [habitatPayload] { [.water, .land, .air] }
+                    public static var allCases: [habitatPayload] { [.water, .land, .air, ._empty] }
                 }
                 public var habitat: Operations.listPets.Input.Query.habitatPayload?
                 /// - Remark: Generated from `#/paths/pets/GET/query/feedsPayload`.