Fix models elements min length

[MonkeyCanCode]

Looking at https://github.com/apache/ossie/blob/main/core-spec/osi-schema.json, from_columns, to_columns, and datasets should not allow empty list (which is the current behavior) as their min is 1 based on the spec:

    "Expression": {
      "type": "object",
      "description": "Expression definition with multi-dialect support",
      "properties": {
        "dialects": {
          "type": "array",
          "items": {
            "$ref": "#/$defs/DialectExpression"
          },
          "minItems": 1
        }
      },

...

    "Relationship": {
      "type": "object",
      "description": "Foreign key relationship between datasets",
...
        "from_columns": {
          "type": "array",
          "items": {
            "type": "string"
          },
          "minItems": 1,
          "description": "Foreign key columns in the 'from' dataset"
        },
        "to_columns": {
          "type": "array",
          "items": {
            "type": "string"
          },
          "minItems": 1,
          "description": "Primary/unique key columns in the 'to' dataset"
        },

...

    "SemanticModel": {
      "type": "object",
      "description": "Top-level container representing a complete semantic model",
...
        "datasets": {
          "type": "array",
          "items": {
            "$ref": "#/$defs/Dataset"
          },
          "minItems": 1,
          "description": "Collection of logical datasets"
        },
...

[xiedeyantu]

Can we leave datasets empty?

[MonkeyCanCode]

From the spec, it should be min 1. Any reason to keep it empty?

[xiedeyantu]

Will we reserve some empty datasets? I'm not entirely sure, but if you're sure about this, I think it's fine.

[MonkeyCanCode]

Per the JSON schema (https://github.com/apache/ossie/blob/main/core-spec/osi-schema.json#L302), datasets has "minItems": 1. A semantic model with no datasets is meaningless. Also, as models are frozen, so we can't do incremental build case as well. I think keep the min of 1 here is more semantic correct as the spec itself doesn't allow empty.