Add null guard before setting context map in Pumper thread

[daniel-mohedano]

Details

Improvement to #3241 with null guard before setting the context map in ThreadedStreamConsumer$Pumper. In the original PR, introduced in 3.5.5, the Pumper constructor captures the parent thread's MDC context map via MDC.getCopyOfContextMap(), and Pumper.run() restores it via MDC.setContextMap(parentThreadsMdcContextMap)

MDC.getCopyOfContextMap() is documented as able to return null values, while MDC.setContextMap() only supports nulled arguments since SLF4J version 2.0.0

When using an adapter conforming to a previous version (for example, Logback <1.4.2), this can cause an NPE, killing the Pumper daemon thread.

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