Guard against null first record and null/blank fieldApiName

[Copilot]

getFirstValue would throw a NullPointerException if the first element of records was null, and could produce unexpected behavior if fieldApiName was null or blank — neither case was handled.

Changes

• Null first-record guard — checks records[0] == null before calling .get(), returning null instead of crashing
• Null/blank fieldApiName guard — uses String.isBlank() to short-circuit before the field lookup
• Both guards are consolidated into a single condition to avoid duplication:

if (firstRecord == null || String.isBlank(req.fieldApiName)) {
    resp.response = null;
    results.add(resp);
    continue;
}

• New tests — testGetFirstValue_NullFirstRecord, testGetFirstValue_NullFieldApiName, testGetFirstValue_BlankFieldApiName