write/service: handle a nil URL from url.Parse without panicking

[c-tonneslan]

Fixes #422. NewClient accepts any string for serverURL and only stores it. A malformed value like a bare host:port (192.168.1.104:8086) makes url.Parse return (nil, err). internal/write.NewService discards that error and immediately calls u.Parse("write") on the nil URL, panicking on first WriteAPIBlocking use.

Stack trace from the repro in the issue:

net/url.(*URL).ResolveReference(0x0, ...)
net/url.(*URL).Parse(0x0, "write")
.../internal/write.NewService(...) service.go:77
.../api.NewWriteAPIBlocking(...)
.../v2.(*clientImpl).WriteAPIBlocking(...)

Fall back to building the write URL by string concatenation when url.Parse fails. The bad address still surfaces later when the HTTP request is dispatched, just as a regular network/HTTP error instead of a panic.

Added a regression test that constructs NewService with the broken URL from the issue.

Fixes #422

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.99%. Comparing base (67f504e) to head (eb81762).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #427   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files          25       25           
  Lines        2326     2326           
=======================================
  Hits         2163     2163           
  Misses        123      123           
  Partials       40       40           

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