Replace if not value.nil? with unless value.nil?

Two adjacent `if not value.nil?` trailing modifiers in
`add_argument`'s XMLTYPE branch used the awkward double negation
form. Replace with `unless value.nil?` for idiomatic Ruby; same
semantics, same line count.

No behavior change. Full suite is green (468 examples, 0 failures,
1 pre-existing pending).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

lib/plsql/procedure_call.rb

@@ -243,8 +243,8 @@ def add_argument(argument, value, argument_metadata = nil)
         when "UNDEFINED", "XMLTYPE", "OPAQUE/XMLTYPE"
           if xmltype_argument?(argument_metadata)
             @declare_sql << "l_#{argument} XMLTYPE;\n"
-            @assignment_sql << "l_#{argument} := XMLTYPE(:#{argument});\n" if not value.nil?
-            @bind_values[argument] = value if not value.nil?
+            @assignment_sql << "l_#{argument} := XMLTYPE(:#{argument});\n" unless value.nil?
+            @bind_values[argument] = value unless value.nil?
             @bind_metadata[argument] = argument_metadata.merge(data_type: "CLOB")
             "l_#{argument}"
           end