Generate synthetic fixtures for nullable types when expansions are requested

[anniel-stripe]

The issue in #445 is caused by a crash when you try to expand a nullable attribute:

$ stripe-mock
stripe-mock 0.159.0
Routing to 306 path(s) and 453 endpoint(s) with 453 validator(s)
Listening for HTTP at address: [::]:12111
Listening for HTTPS at address: [::]:12112
Request: GET /v1/invoices/in_1NhTSDJ7Bu9fw2TI7tY1f5zD
Request data = map[expand:[account_tax_ids]]
2023/08/25 15:27:12 http: panic serving 127.0.0.1:53788: Responding to GET /v1/invoices/in_1NhTSDJ7Bu9fw2TI7tY1f5zD:
Dereferencing '#/components/schemas/invoice':
Using fixture 'invoice':
In property 'account_tax_ids' of object:
We were asked to expand a key, but our example has null for that key.
goroutine 49 [running]:
net/http.(*conn).serve.func1()
	/opt/hostedtoolcache/go/1.19.7/x64/src/net/http/server.go:1850 +0xb0
panic({0x100e9c660, 0x140018e3560})
	/opt/hostedtoolcache/go/1.19.7/x64/src/runtime/panic.go:890 +0x258
github.com/stripe/stripe-mock/server.(*DataGenerator).generateInternal(0x140010398f8, 0x14001acf320)
	/home/runner/work/stripe-mock/stripe-mock/server/generator.go:339 +0xe2c
github.com/stripe/stripe-mock/server.(*DataGenerator).generateInternal(0x140010398f8, 0x14001039670)
	/home/runner/work/stripe-mock/stripe-mock/server/generator.go:399 +0x10dc
github.com/stripe/stripe-mock/server.(*DataGenerator).Generate(0x140010398f8, 0x14001039950)
	/home/runner/work/stripe-mock/stripe-mock/server/generator.go:105 +0x114
github.com/stripe/stripe-mock/server.(*StubServer).HandleRequest(0x140008ea260, {0x100f044a8, 0x1400081ba40}, 0x14001ab0000)
	/home/runner/work/stripe-mock/stripe-mock/server/server.go:353 +0x968
net/http.HandlerFunc.ServeHTTP(0x14001ab9a38?, {0x100f044a8?, 0x1400081ba40?}, 0x100d6f7e8?)
	/opt/hostedtoolcache/go/1.19.7/x64/src/net/http/server.go:2109 +0x38
net/http.(*ServeMux).ServeHTTP(0x140003dc0a4?, {0x100f044a8, 0x1400081ba40}, 0x14001ab0000)
	/opt/hostedtoolcache/go/1.19.7/x64/src/net/http/server.go:2487 +0x140
github.com/stripe/stripe-mock/server.(*DoubleSlashFixHandler).ServeHTTP(0x140018a29c0, {0x100f044a8, 0x1400081ba40}, 0x14001ab0000)
	/home/runner/work/stripe-mock/stripe-mock/server/server.go:47 +0xb0
net/http.serverHandler.ServeHTTP({0x14001a9a1b0?}, {0x100f044a8, 0x1400081ba40}, 0x14001ab0000)
	/opt/hostedtoolcache/go/1.19.7/x64/src/net/http/server.go:2947 +0x2c4
net/http.(*conn).serve(0x14001aaa000, {0x100f049a0, 0x14001a9a0c0})
	/opt/hostedtoolcache/go/1.19.7/x64/src/net/http/server.go:1991 +0x560
created by net/http.(*Server).Serve
	/opt/hostedtoolcache/go/1.19.7/x64/src/net/http/server.go:3102 +0x444

This is because generateSyntheticFixtures defaults to returning nil if the property is nullable. However, this poses an issue when we want to expand that nullable field.

This PR updates generateSyntheticFixtures to check expansions and generate non-nil fixtures for nullable properties if they're being expanded.