Generation panic on custom string

[nesty156]

Describe the bug
When running the swag v2 with this flags: swag init -v3.1 --parseDependency --parseInternal --parseDepth 1 -g main.go I get panic because of custom string

type ThreatType string

const (
	MALWARE               ThreatType = "MALWARE"
	SPYWARE               ThreatType = "SPYWARE"
)

type Threat struct {
	ThreatType ThreatType `json:"threatType" form:"threatType"`
}

To Reproduce
Steps to reproduce the behavior:

1. Install swag v2
2. Run the docs generation: swag init -v3.1 --parseDependency --parseInternal --parseDepth 1 -g main.go

Expected behavior
No panic :)

Screenshots
If applicable, add screenshots to help explain your problem.

Your swag version
e.g. 2.0.0

Your go version
e.g. 1.25.0

Desktop (please complete the following information):

• OS: macOS 15.7.2

Additional context
This issue #2078 has the same problem