GH-45700: [C++][Compute] Added nullptr check in Equals method to handle null impl_ pointers (#45701)

### Rationale for this change

### What changes are included in this PR?
This PR adds a check to ensure that the Expression is not nullptr
### Are these changes tested?
Yes.
### Are there any user-facing changes?
No

* GitHub Issue: #45700

Lead-authored-by: mroz45 <kamil.tokarek@secom.com.pl>
Co-authored-by: kamilt <kamil.tokarek@secom.com.pl>
Co-authored-by: Rossi Sun <zanmato1984@gmail.com>
Signed-off-by: Rossi Sun <zanmato1984@gmail.com>

Author: mroz45

cpp/src/arrow/compute/expression.cc

@@ -221,6 +221,8 @@ void PrintTo(const Expression& expr, std::ostream* os) {
 bool Expression::Equals(const Expression& other) const {
   if (Identical(*this, other)) return true;
 
+  if (impl_ == nullptr || other.impl_ == nullptr) return false;
+
   if (impl_->index() != other.impl_->index()) {
     return false;
   }

cpp/src/arrow/compute/expression_test.cc

@@ -338,6 +338,15 @@ TEST(Expression, Equality) {
               literal(std::numeric_limits<double>::signaling_NaN()));
   }
 
+  // Equality when underlying `impl_` is null.
+  Expression expr1;
+  Expression expr2(literal(1));
+  EXPECT_NE(literal("a"), expr1);
+  EXPECT_NE(expr1, literal("a"));
+
+  EXPECT_FALSE(expr1.Equals(expr2));
+  EXPECT_FALSE(expr2.Equals(expr1));
+
   EXPECT_EQ(field_ref("a"), field_ref("a"));
   EXPECT_NE(field_ref("a"), field_ref("b"));
   EXPECT_NE(field_ref("a"), literal(2));