vdbe: modified the Null instruction

modified the Null instruction to more closely match SQLite semantics. Allows passing in a second register and all registers from r1..r2 area set to null

core/translate/mod.rs

@@ -224,7 +224,7 @@ fn emit_schema_entry(
     if let Some(sql) = sql {
         program.emit_string8(sql, sql_reg);
     } else {
-        program.emit_null(sql_reg);
+        program.emit_null(sql_reg, None);
     }
 
     let record_reg = program.alloc_register();
@@ -552,8 +552,8 @@ fn translate_drop_table(
     let init_label = program.emit_init();
     let start_offset = program.offset();
 
-    let null_reg = program.alloc_register(); //  r1
-    program.emit_null(null_reg);
+    let r1 = program.alloc_register(); //  r1
+    program.emit_null(r1, None);
     let tbl_name_reg = program.alloc_register(); //  r2
     let table_reg = program.emit_string8_new_reg(tbl_name.name.0.clone()); //  r3
     program.mark_last_insn_constant();
@@ -573,6 +573,7 @@ fn translate_drop_table(
     });
     program.emit_insn(Insn::OpenWriteAwait {});
 
+    //  1. Remove all entries from the schema table related to the table we are dropping, except for triggers
     //  loop to beginning of schema table
     program.emit_insn(Insn::RewindAsync {
         cursor_id: sqlite_schema_cursor_id,
@@ -638,6 +639,10 @@ fn translate_drop_table(
         is_temp: 0,
     });
 
+    let r6 = program.alloc_register();
+    let r7 = program.alloc_register();
+    program.emit_null(r6, Some(r7));
+
     //  end of the program
     program.emit_halt();
     program.resolve_label(init_label, program.offset());

core/translate/pragma.rs

@@ -233,7 +233,7 @@ fn query_pragma(
                     // dflt_value
                     match &column.default {
                         None => {
-                            program.emit_null(base_reg + 4);
+                            program.emit_null(base_reg + 4, None);
                         }
                         Some(expr) => {
                             program.emit_string8(expr.to_string(), base_reg + 4);

core/vdbe/builder.rs

@@ -138,11 +138,8 @@ impl ProgramBuilder {
         });
     }
 
-    pub fn emit_null(&mut self, dest: usize) {
-        self.emit_insn(Insn::Null {
-            dest,
-            dest_end: None,
-        });
+    pub fn emit_null(&mut self, dest: usize, dest_end: Option<usize>) {
+        self.emit_insn(Insn::Null { dest, dest_end });
     }
 
     pub fn emit_result_row(&mut self, start_reg: usize, count: usize) {