Empty header key appears if user set Custom HTTP Headers as empty fields when create datasource

[dmitryk-dk]

What happened:
While configuring the data source, the user can define empty custom headers like in the image below

In that case, opts.Header will have empty header keys, which are not allowed

and when you try to query the data source, you will get an error like

Post "https://play-vmlogs.victoriametrics.com/select/logsql/query?end=1737573753&limit=1000&query=%2A&start=1737570153": net/http: invalid header field name ""

What you expected to happen:
Headers keys shouldn't be empty and query works as expected

How to reproduce it (as minimally and precisely as possible):
Please check my description of the issue

Anything else we need to know?:

Environment:

• SDK version: v0.260.3
• Grafana version: v11.3.0
• Go version: v1.23.4