Fix empty string handling for TruncatingBufferedWriter

[bayne]

ERROR StatusConsoleListener An exception occurred processing Appender console
 java.lang.IndexOutOfBoundsException: invalid offset: 0
	at org.apache.logging.log4j.layout.template.json.util.TruncatingBufferedWriter.write(TruncatingBufferedWriter.java:131)
	at java.base/java.io.PrintWriter.write(PrintWriter.java:541)
	at java.base/java.io.PrintWriter.write(PrintWriter.java:558)
	at java.base/java.io.PrintWriter.print(PrintWriter.java:685)
	at com.lowagie.text.ExceptionConverter.printStackTrace(ExceptionConverter.java:162)
	at org.apache.logging.log4j.layout.template.json.resolver.StackTraceStringResolver.resolve(StackTraceStringResolver.java:80)
	at org.apache.logging.log4j.layout.template.json.resolver.ExceptionResolver.lambda$createStackTraceStringResolver$3(ExceptionResolver.java:302)
	at org.apache.logging.log4j.layout.template.json.resolver.ExceptionResolver.resolve(ExceptionResolver.java:445)
	at org.apache.logging.log4j.layout.template.json.resolver.ExceptionResolver.resolve(ExceptionResolver.java:188)
	at org.apache.logging.log4j.layout.template.json.resolver.TemplateResolver.resolve(TemplateResolver.java:66)
	at org.apache.logging.log4j.layout.template.json.resolver.TemplateResolvers$4.resolve(TemplateResolvers.java:284)
...

The client code assumes empty strings will not result in an exception being thrown

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (if it fails due to code formatting issues reported by Spotless, simply run ./mvnw spotless:apply and retry)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests for the changes are provided
• Commits are signed (optional, but highly recommended)

[vy]

@bayne, thanks so much for the report, fix, and tests! Much appreciated! 🙇

I removed if (end == 0) along with a test case, since it is making append() unnecessarily more lenient and breaking a test.

Could you also create a /src/changelog/.2.x.x/fix-TruncatingBufferedWriter-empty-input-handling.xml, please? (You can copy 2379_fix_message_factory_properties.xml and adapt it.)