Describe the bug
A clear and concise description of what the bug is.
goctll api swagger panic with nil pointer when api tag is wrong
To Reproduce
Steps to reproduce the behavior, if applicable:
write a api file with wrong field tag, such as:
endLine int form:"endLine",optional,default=100"

1. The code is

type FileContentRequest {
clientId string form:"clientId" // 用户机器ID
codebasePath string form:"codebasePath" // 项目绝对路径
filePath string form:"filePath" // 文件相对路径
startLine int form:"startLine,optional,default=1" // 开始行（默认1）
endLine int form:"endLine",optional,default=100" // 结束行（默认100，-1=全部）
}

`form:"endLine",optional,default=100"`    this  is  wrong  tag,   when  correct  it,   cmd  run ok.
2. The error is

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x139e697]

goroutine 1 [running]:
github.com/zeromicro/go-zero/tools/goctl/api/spec.(*Tags).Get(...)
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/spec/tags.go:52
github.com/zeromicro/go-zero/tools/goctl/api/swagger.isRequired({0x50?, 0xa7?, {0x9?, 0x26833a0?}}, 0x0)
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/swagger/swagger.go:221 +0x17
github.com/zeromicro/go-zero/tools/goctl/api/swagger.rangeMemberAndDo({0x0?, 0x0?, {0x0?, 0x0?}}, {0x2683370?, 0xc0004bb360?}, 0xc0004bb3a0)
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/swagger/swagger.go:215 +0xd2
github.com/zeromicro/go-zero/tools/goctl/api/swagger.parametersFromType({0xf8?, 0xb5?, {0x0?, 0x4?}}, {0xc00027fb40, 0x3}, {0x2683370, 0xc000312b40})
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/swagger/parameter.go:43 +0x268
github.com/zeromicro/go-zero/tools/goctl/api/swagger.spec2Path({, , {, }}, {{}, {, _, _}}, {{0x0}, {0xc00027fb40, ...}, ...})
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/swagger/path.go:90 +0x44c
github.com/zeromicro/go-zero/tools/goctl/api/swagger.spec2Paths({0x10?, 0x62?, {0x0?, 0x8?}}, {{0xc000296200, 0xf}, {0xc00039a180, 0x3, 0x4}})
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/swagger/path.go:24 +0x39d
github.com/zeromicro/go-zero/tools/goctl/api/swagger.spec2Swagger(0xc0000dc690)
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/swagger/swagger.go:32 +0x3da
github.com/zeromicro/go-zero/tools/goctl/api/swagger.Command(0xc000396100?, {0x23fbcd2?, 0x4?, 0x23fbbd6?})
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/api/swagger/command.go:49 +0xb9
github.com/spf13/cobra.(*Command).execute(0xc000394908, {0xc0000b4180, 0x6, 0x6})
D:/programs/gopath/pkg/mod/github.com/spf13/[email protected]/command.go:1015 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0xc0002dc908)
D:/programs/gopath/pkg/mod/github.com/spf13/[email protected]/command.go:1148 +0x46f
github.com/spf13/cobra.(*Command).Execute(...)
D:/programs/gopath/pkg/mod/github.com/spf13/[email protected]/command.go:1071
github.com/zeromicro/go-zero/tools/goctl/cmd.Execute()
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/cmd/root.go:47 +0x6c
main.main()
D:/programs/gopath/pkg/mod/github.com/zeromicro/go-zero/tools/[email protected]/goctl.go:12 +0x48

**Expected behavior**
A clear and concise description of what you expected to happen.
run ok    or   tell me   what's wrong.
**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environments (please complete the following information):**
- OS: window
- go-zero 1.8.3
- goctl version 1.8.3

**More description**
Add any other context about the problem here.