sqlite3: fix internal return type violation in escapeString() (#22026)

If this call fails due to an internal libsqlite3 error, then the
function will return NULL (as that's the default value set by the VM).
However, the function is marked with a non-nullable string return type.
Therefore this will result in a type violation and a fatal error in
debug mode.
Either we solve it by making the function nullable or throw.
I chose the latter as it is less of a footgun.

ext/sqlite3/sqlite3.c

@@ -478,6 +478,9 @@ PHP_METHOD(SQLite3, escapeString)
 		if (ret) {
 			RETVAL_STRING(ret);
 			sqlite3_free(ret);
+		} else {
+			zend_throw_exception_ex(php_sqlite3_exception_ce, 0, "Unable to escape string");
+			RETURN_THROWS();
 		}
 	} else {
 		RETURN_EMPTY_STRING();