fix: only access propertiesSchema.properties once it's confirmed non-undefined

[TWiStErRob]

Description 📝

running with --debug gave

Trace: {
  isWrappedError: true,
  description: 'An unexpected error occurred.',
  notes: [ "Cannot read properties of undefined (reading 'properties')" ],
  error: TypeError: Cannot read properties of undefined (reading 'properties')
      at getPropertiesSchema (rudder-typer/src/generators/ast.js:45:45)
      at rudder-typer/src/generators/android/android.js:75:32
      at Generator.next (<anonymous>)
      at rudder-typer/src/generators/android/android.js:7:71
      at new Promise (<anonymous>)
      at __awaiter (rudder-typer/src/generators/android/android.js:3:12)
      at Object.generateTrackCall (rudder-typer/src/generators/android/android.js:74:77)
      at rudder-typer/src/generators/gen.js:236:37
      at Generator.next (<anonymous>)
      at fulfilled (rudder-typer/src/generators/gen.js:4:58)
}
    at ErrorBoundary.<anonymous> (rudder-typer/src/cli/commands/error.js:51:25)
    at Generator.next (<anonymous>)
    at rudder-typer/src/cli/commands/error.js:7:71
    at new Promise (<anonymous>)
    at __awaiter (rudder-typer/src/cli/commands/error.js:3:12)
    at ErrorBoundary.reportError (rudder-typer/src/cli/commands/error.js:49:40)
    at ErrorBoundary.<anonymous> (rudder-typer/src/cli/commands/error.js:63:24)
    at Generator.next (<anonymous>)
    at rudder-typer/src/cli/commands/error.js:7:71
    at new Promise (<anonymous>)

Modifying the code to debug, it crashed when event was this:

{
	"name": "identify",
	"type": 5,
	"properties": [
		{
			"name": "traits",
			"type": 5,
			"properties": [
				{
					"name": "value",
					"type": 4,
					"description": "...",
					"isRequired": true
				}
			]
		}
	],
	"description": "My custom identify"
}

in the plan.json this was represented by:

{
	"rules": {
		"events": [
			{
				"description": "My custom identify",
				"eventType": "identify",
				"name": "",
				"rules": {
					"$defs": {
					},
					"$schema": "http://json-schema.org/draft-07/schema#",
					"description": "My custom identify",
					"properties": {
						"traits": {
							"additionalProperties": false,
							"properties": {
								"value": {
									"description": "...",
									"type": [
										"number"
									]
								}
							},
							"required": [
								"value"
							],
							"type": "object"
						}
					},
					"type": "object"
				}
			},

Please add tests for this case.

[contributor-support]

Thank you @TWiStErRob for contributing this PR.
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