nil pointer during url parsing

[breadly7]

Describe the bug
When starting the exporter (from version 0.42.0 on) and providing a password that contains special characters the exporter crashes during the call here:
https://github.com/percona/mongodb_exporter/blob/v0.42.0/main.go#L147

Using an older version (e.g. 0.41.0) with the same arguments results in an error that logs the mongodb connection string.

Are passwords with special characters even supported?

To Reproduce

($PASSWORDS contains special characters, in my tests I used ? and !)

1. Start the exporter with the following arguments: mongodb_exporter --mongodb.uri "mongodb://localhost:27017/?authSource=admin" --mongodb.user $USER --mongodb.password $PASSWORD
2. The exporter crashes pretty much immediately

Expected behavior
I expect it not to crash.

Logs
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0xcea293]
goroutine 1 [running]:
net/url.(*URL).Port(...)
/opt/bitnami/go/src/net/url/url.go:1196
main.buildExporter({{0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0xc0006026f0, 0x1, ...}, ...}, ...)
/bitnami/blacksmith-sandox/mongodb-exporter-0.42.0/src/github.com/percona/mongodb_exporter/main.go:147 +0xd3
main.buildServers({{0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0xc0006026f0, 0x1, ...}, ...}, ...)
/bitnami/blacksmith-sandox/mongodb-exporter-0.42.0/src/github.com/percona/mongodb_exporter/main.go:197 +0x105
main.main()
/bitnami/blacksmith-sandox/mongodb-exporter-0.42.0/src/github.com/percona/mongodb_exporter/main.go:138 +0x725

Environment

• environment: k8s
• exporter version: 0.42.0

Additional context