[BUG] [typescript-angular] Invalid string enum mapping for `Null` value

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

I have the following enum definition:

EdmExpressionKind:
  enum:
    - 0
    - 1
    - 2
    - 3
    - 4
    - 5
    - 6
    - 7
    - 8
    - 9
    - 10
    - 11
    - 12
    - 13
    - 14
    - 15
    - 16
    - 17
    - 18
    - 19
    - 20
    - 21
    - 22
    - 23
    - 24
    - 25
  type: integer
  format: int32
  x-enum-varnames:
    - None
    - BinaryConstant
    - BooleanConstant
    - DateTimeOffsetConstant
    - DecimalConstant
    - FloatingConstant
    - GuidConstant
    - IntegerConstant
    - StringConstant
    - DurationConstant
    - Null
    - Record
    - Collection
    - Path
    - If
    - Cast
    - IsType
    - FunctionApplication
    - LabeledExpressionReference
    - Labeled
    - PropertyPath
    - NavigationPropertyPath
    - DateConstant
    - TimeOfDayConstant
    - EnumMember
    - AnnotationPath

typescript-angular generator produces the following model:

export enum EdmExpressionKind {
    None = 0,
    BinaryConstant = 1,
    BooleanConstant = 2,
    DateTimeOffsetConstant = 3,
    DecimalConstant = 4,
    FloatingConstant = 5,
    GuidConstant = 6,
    IntegerConstant = 7,
    StringConstant = 8,
    DurationConstant = 9,
     = 10,
    Record = 11,
    Collection = 12,
    Path = 13,
    If = 14,
    Cast = 15,
    IsType = 16,
    FunctionApplication = 17,
    LabeledExpressionReference = 18,
    Labeled = 19,
    PropertyPath = 20,
    NavigationPropertyPath = 21,
    DateConstant = 22,
    TimeOfDayConstant = 23,
    EnumMember = 24,
    AnnotationPath = 25
};

So it fails to properly map Null = 10 value and produces invalid typescript file

openapi-generator version

5.3.0

OpenAPI declaration file content fragment

components:
  schemas:
    EdmExpressionKind:
      enum:
        - 0
        - 1
        - 2
        - 3
        - 4
        - 5
        - 6
        - 7
        - 8
        - 9
        - 10
        - 11
        - 12
        - 13
        - 14
        - 15
        - 16
        - 17
        - 18
        - 19
        - 20
        - 21
        - 22
        - 23
        - 24
        - 25
      type: integer
      format: int32
      x-enum-varnames:
        - None
        - BinaryConstant
        - BooleanConstant
        - DateTimeOffsetConstant
        - DecimalConstant
        - FloatingConstant
        - GuidConstant
        - IntegerConstant
        - StringConstant
        - DurationConstant
        - Null
        - Record
        - Collection
        - Path
        - If
        - Cast
        - IsType
        - FunctionApplication
        - LabeledExpressionReference
        - Labeled
        - PropertyPath
        - NavigationPropertyPath
        - DateConstant
        - TimeOfDayConstant
        - EnumMember
        - AnnotationPath
      x-import-mapping: Microsoft.OData.Edm.EdmExpressionKind

Generation Details

@openapitools/openapi-generator-cli generate -i swagger.yaml -g typescript-angular -o output --type-mappings AnyType=any --additional-properties stringEnums=true

Steps to reproduce

Related issues/PRs

Suggest a fix

Start with the typescript-angular generator using the provided OpenAPI enum and generation command, then trace how x-enum-varnames are converted into the shown TypeScript enum. Reproduce the empty member for Null and verify that generation produces valid TypeScript with Null = 10 preserved.