fix: null-check AppenderDynamicMBean setLayout instantiation

[SebTardif]

What Problem This Solves

AppenderDynamicMBean.invoke("setLayout") calls OptionConverter.instantiateByClassName(..., null) and immediately passes the result to appender.setLayout(...). When the class name is invalid or not a Layout, instantiation returns null and the next line NPEs. Same pattern as LoggerDynamicMBean.addAppender (#4185).

Evidence

Failure scenario

final Layout layout =
    (Layout) OptionConverter.instantiateByClassName((String) params[0], Layout.class, null);
appender.setLayout(layout); // NPE when instantiate returns null

Red-green / tests

export JAVA_HOME=$(/usr/libexec/java_home -v 17)
./mvnw -pl log4j-1.2-api -am test \
  -Dtest=AppenderDynamicMBeanTest \
  -Dsurefire.failIfNoSpecifiedTests=false

• Invalid class: no NPE, layout remains null
• Valid PatternLayout: layout attached

Sibling audit

• LoggerDynamicMBean.addAppender: already guarded in fix: null-check LoggerDynamicMBean addAppender instantiation #4185
• registerLayoutMBean: already null-safe
• No other instantiateByClassName + immediate dereference in log4j-1.2-api JMX package

Summary

Null-check + error log on failed layout instantiation; unit tests + changelog.