Panic on metrics request when config file is empty

[hhromic]

Describe the bug
When the config file is empty, the application starts normally but then panics on a request to /metrics:

ts=2023-01-19T12:20:27.778Z 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=2023-01-19T12:20:27.778Z caller=klog.go:84 level=debug func=Infof msg="Loading configuration from sql-exporter.yaml"
ts=2023-01-19T12:20:27.778Z caller=klog.go:96 level=warn func=Warning msg="Listening on :9399"
ts=2023-01-19T12:20:27.779Z caller=tls_config.go:232 level=info msg="Listening on" address=[::]:9399
ts=2023-01-19T12:20:27.779Z caller=tls_config.go:235 level=info msg="TLS is disabled." http2=false address=[::]:9399

// A request to `/metrics` is done here

2023/01/19 12:20:36 http: panic serving 172.17.0.1:43220: runtime error: invalid memory address or nil pointer dereference
goroutine 30 [running]:
net/http.(*conn).serve.func1()
        /usr/local/go/src/net/http/server.go:1850 +0xbf
panic({0x10e49a0, 0x1c74650})
        /usr/local/go/src/runtime/panic.go:890 +0x262
main.contextFor(0xc0004d8100, {0x1467240, 0xc0004b58c0})
        /app/cmd/sql_exporter/promhttp.go:77 +0x3c
main.ExporterHandlerFor.func1({0x7f2ebd6f93b8, 0xc0004b26e0}, 0xc0004d8100)
        /app/cmd/sql_exporter/promhttp.go:32 +0x72
net/http.HandlerFunc.ServeHTTP(0x11208a0?, {0x7f2ebd6f93b8?, 0xc0004b26e0?}, 0xff4bb8?)
        /usr/local/go/src/net/http/server.go:2109 +0x2f
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerInFlight.func1({0x7f2ebd6f93b8, 0xc0004b26e0}, 0x1466100?)
        /go/pkg/mod/github.com/prometheus/client_golang@v1.14.0/prometheus/promhttp/instrument_server.go:60 +0xd4
net/http.HandlerFunc.ServeHTTP(0x1466188?, {0x7f2ebd6f93b8?, 0xc0004b26e0?}, 0x0?)
        /usr/local/go/src/net/http/server.go:2109 +0x2f
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerCounter.func1({0x1466188?, 0xc000682000?}, 0xc0004d8100)
        /go/pkg/mod/github.com/prometheus/client_golang@v1.14.0/prometheus/promhttp/instrument_server.go:146 +0xb8
net/http.HandlerFunc.ServeHTTP(0xc00012daf0?, {0x1466188?, 0xc000682000?}, 0x0?)
        /usr/local/go/src/net/http/server.go:2109 +0x2f
net/http.(*ServeMux).ServeHTTP(0x0?, {0x1466188, 0xc000682000}, 0xc0004d8100)
        /usr/local/go/src/net/http/server.go:2487 +0x149
net/http.serverHandler.ServeHTTP({0xc0004b5fb0?}, {0x1466188, 0xc000682000}, 0xc0004d8100)
        /usr/local/go/src/net/http/server.go:2947 +0x30c
net/http.(*conn).serve(0xc0004c4b40, {0x1466da8, 0xc0004b5ec0})
        /usr/local/go/src/net/http/server.go:1991 +0x607
created by net/http.(*Server).Serve
        /usr/local/go/src/net/http/server.go:3102 +0x4db

To Reproduce
Steps to reproduce the behavior:

• Start the exporter using a blank/empty config file
• Do a request to /metrics
• See panic

Expected behavior
The application should not start with a blank/empty config file as one of jobs or target is required.

Configuration
Just an empty file is sufficient to trigger this panic.

Additional context
It can be argued that a blank/empty config file is not a use-case, but the application should not panic regardless. A (mistakingly) invalid config file could be a very possible situation and a proper error messages is better than a panic.

Probably is related to #182 in the way the YAML unmarshalling is performed.