isogram: false positive on variable declaration with zero value

False positive on solution: https://exercism.io/mentor/solutions/6356bab9214544fe85b37b0c646c183b?iteration_idx=3

```go
package isogram

import (
"strings"
"unicode"
)

//IsIsogram returns true if string is isogram
func IsIsogram(word string) bool {
wordUpper := strings.ToUpper(word)
for i := 0; i < len(wordUpper); i++ {
if unicode.IsLetter(rune(wordUpper[i])) {
if strings.Count(wordUpper, string(wordUpper[i])) > 1 {
return false
}
}
}
return true
}
```

Wrong output:
- If you're declaring a variable which can be initialized with its [zero value](https://golang.org/ref/spec#The_zero_value), then `var s string` is more idiomatic than an assignment: `s := ""`.

Start by reproducing the report with the Go solution and the zero-value declaration shown in the issue, then search the analyzer for the emitted wording. Done means the analyzer no longer reports this message for the shown valid declaration.

Written by the indexing model from the issue text.