Panic on start when `global` key is missing in config file

[hhromic]

Describe the bug

When the global key in the main config file is missing, the application crashes with a panic:

ts=2023-01-19T12:19:18.220Z caller=klog.go:108 level=warn func=Warningf msg="Starting SQL exporter (version=0.9.2, branch=HEAD, revision=e1a5775cca58bf1110516baebc6fe7854d18279b) (go=go1.19.4, platform=linux/amd64, user=root@46fa4bc8dec6, date=20221222-15:10:06)"
ts=2023-01-19T12:19:18.220Z caller=klog.go:84 level=debug func=Infof msg="Loading configuration from sql-exporter.yaml"
ts=2023-01-19T12:19:18.221Z caller=klog.go:84 level=debug func=Infof msg="Loaded collector 'tables' from collectors/tables.yaml"
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x8ced68]

goroutine 1 [running]:
gopkg.in/yaml%2ev3.handleErr(0xc0005abbd0)
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/yaml.go:294 +0x6d
panic({0x10e49a0, 0x1c74650})
        /usr/local/go/src/runtime/panic.go:884 +0x212
github.com/burningalchemist/sql_exporter/config.(*Config).UnmarshalYAML(0xc0001d2000, 0x112ed60?)
        /app/config/config.go:76 +0x168
gopkg.in/yaml%2ev3.(*decoder).callObsoleteUnmarshaler(0xc0001d2070, 0xc0004bc1e0, {0x7fc5cd458ab0, 0xc0001d2000})
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/decode.go:379 +0xa7
gopkg.in/yaml%2ev3.(*decoder).prepare(0x0?, 0x0?, {0x11d7f40?, 0xc0001d2000?, 0x0?})
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/decode.go:427 +0x245
gopkg.in/yaml%2ev3.(*decoder).unmarshal(0xc0001d2070, 0xc0004bc1e0, {0x11d7f40?, 0xc0001d2000?, 0x0?})
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/decode.go:502 +0x2a5
gopkg.in/yaml%2ev3.(*decoder).document(0x0?, 0x0?, {0x11d7f40?, 0xc0001d2000?, 0xc0002c1800?})
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/decode.go:527 +0x5b
gopkg.in/yaml%2ev3.(*decoder).unmarshal(0xc0001d2070, 0xc0004bc140, {0x11d7f40?, 0xc0001d2000?, 0x7fc5f4954a68?})
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/decode.go:498 +0x42d
gopkg.in/yaml%2ev3.unmarshal({0xc0004c0000, 0xf1, 0x200}, {0x112ed60?, 0xc0001d2000}, 0x50?)
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/yaml.go:167 +0x426
gopkg.in/yaml%2ev3.Unmarshal(...)
        /go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/yaml.go:89
github.com/burningalchemist/sql_exporter/config.Load({0x7fff28047e35, 0x12})
        /app/config/config.go:29 +0x115
github.com/burningalchemist/sql_exporter.NewExporter({0x7fff28047e35?, 0x1b?})
        /app/exporter.go:41 +0x33
main.main()
        /app/cmd/sql_exporter/main.go:80 +0x529

EDIT: I pasted the wrong panic before, now it is correct.

To Reproduce
Steps to reproduce the behavior:

1. Try to start the exporter using the config shown in Configuration. Make sure at least one collector file is present.
2. See panic

Expected behavior
In the example file below, it is expected that all the configuration options under global are set to their defaults.

Configuration

target:
  data_source_name: 'driver://user:pass@dbhost:5433/dbname'
  collectors: [example]
collector_files: ['collectors/*.yaml']

Additional context
The panic can be avoided (and defaults for globals applied) by defining an empty-object global: {} key in the config:

global: {}
target:
  data_source_name: 'driver://user:pass@dbhost:5433/dbname'
  collectors: [example]
collector_files: ['collectors/*.yaml']