Fix DBNull description: missing value vs. nonexistent column (#51951)

* Initial plan

* Fix DBNull description: "missing value in a database column" not "nonexistent database column"

Co-authored-by: BillWagner <493969+BillWagner@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: BillWagner <493969+BillWagner@users.noreply.github.com>

Author: Copilot

docs/visual-basic/language-reference/nothing.md

@@ -45,7 +45,7 @@ If you declare a variable without using an `As` clause and set it to `Nothing`,
 
 When you assign `Nothing` to an object variable, it no longer refers to any object instance. If the variable had previously referred to an instance, setting it to `Nothing` does not terminate the instance itself. The instance is terminated, and the memory and system resources associated with it are released, only after the garbage collector (GC) detects that there are no active references remaining.
 
-`Nothing` differs from the <xref:System.DBNull> object, which represents an uninitialized variant or a nonexistent database column.
+`Nothing` differs from the <xref:System.DBNull> object, which represents an uninitialized variant or a missing value in a database column.
 
 ## See also