Invalid interface conversion in Swift cataloger

[elifarley]

What happened:
When parsing an empty file named Package.resolved (for Swift / iOS), I got an error message:

1 error occurred:
	* interface conversion: interface {} is nil, not float64 at:
goroutine 84 [running]:
runtime/debug.Stack()
	/opt/hostedtoolcache/go/1.21.1/x64/src/runtime/debug/stack.go:24 +0x5e
github.com/anchore/syft/syft/pkg/cataloger.runCataloger.func1()
	/home/runner/work/syft/syft/syft/pkg/cataloger/catalog.go:57 +0x3d
panic({0x15f97a0?, 0xc0004c36b0?})
	/opt/hostedtoolcache/go/1.21.1/x64/src/runtime/panic.go:914 +0x21f
github.com/anchore/syft/syft/pkg/cataloger/swift.parsePackageResolved({0x1?, 0xc0004a3728?}, 0x453689?, {{{{{...}, {...}}, {0x0, 0x0}, {0x2, {...}}}, {0xc0004c34a0}}, ...})
	/home/runner/work/syft/syft/syft/pkg/cataloger/swift/parse_package_resolved.go:70 +0x538
github.com/anchore/syft/syft/pkg/cataloger/generic.(*Cataloger).Catalog(0xc0002ce810, {0x1db7638, 0xc0001ce360})
	/home/runner/work/syft/syft/syft/pkg/cataloger/generic/cataloger.go:129 +0x76e
github.com/anchore/syft/syft/pkg/cataloger.runCataloger({0x1da9240, 0xc0002ce810}, {0x1db7638?, 0xc0001ce360})
	/home/runner/work/syft/syft/syft/pkg/cataloger/catalog.go:65 +0x1ea
github.com/anchore/syft/syft/pkg/cataloger.Catalog.func1()
	/home/runner/work/syft/syft/syft/pkg/cataloger/catalog.go:146 +0xdc
created by github.com/anchore/syft/syft/pkg/cataloger.Catalog in goroutine 82
	/home/runner/work/syft/syft/syft/pkg/cataloger/catalog.go:141 +0x345

What you expected to happen:
No error would have happened.

An empty file should have been simply skipped, as there's nothing interesting in it.

Steps to reproduce the issue:

mkdir /tmp/syft-error
touch /tmp/syft-error/Package.resolved
syft packages dir:/tmp/syft-error 2>/dev/null

Anything else we need to know?:

Environment:

• Output of syft version: 0.93.0
• OS (e.g: cat /etc/os-release or similar): Ubuntu 22.04.3 LTS