fix: StringIsNotEmptyAssertion message says 'not empty or whitespace' but only checks IsNullOrEmpty

[thomhurst]

Description

File: TUnit.Assertions/Conditions/StringAssertions.cs (Lines 506-514)

The StringIsNotEmptyAssertion expectation message says "to not be empty or whitespace" but the code only checks !string.IsNullOrEmpty(value). This allows whitespace-only strings like " " to pass.

Impact

Misleading — developers might assume .IsNotEmpty() rejects whitespace strings.

Suggested Fix

Either:

1. Fix the expectation message to say "to not be null or empty" (matching actual behavior)
2. Or implement .IsNotNullOrWhitespace() as a separate assertion and fix this one's message