[CALCITE-6817] Add string representation of default nulls direction for RelNode #14776

Job	Run time
Windows (JDK 17)	14m 48s
Windows (JDK 8)	15m 52s
Linux (JDK 8, oldest Guava, America/New_York Timezone)	10m 56s
Windows (JDK 21)	15m 47s
Linux (JDK 11, Pacific/Chatham Timezone)	10m 24s
Linux (JDK 8, latest Guava, America/New_York Timezone)	12m 25s
Linux (JDK 21)	10m 11s
Linux (JDK 17)	10m 42s
Linux (JDK 11, Avatica main)	14m 24s
Linux (JDK 23)	12m 11s
macOS (JDK 21)	49m 38s
CheckerFramework (JDK 11)	10m 50s
ErrorProne (JDK 11, latest Guava)	4m 21s
CheckerFramework (JDK 11, oldest Guava)	11m 2s
Druid Tests	6m 49s
Linux (JDK 8) Slow Tests	0s
	3h 30m 20s

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