feat: empty init() function is present by default

pages/book/contracts.mdx

@@ -80,9 +80,6 @@ contract Example {
     const StateDelivered: Int = 2;
     const StateDisputed: Int = 3;
 
-    // no need to init constants
-    init() {}
-
     get fun sum(): Int {
         // access constants from anywhere
         return GlobalConst1 + self.ContractConst1 + self.StatePaid;
@@ -111,6 +108,14 @@ contract Example {
 }
 ```
 
+If a contract doesn't have any persistent state variables, or they all have their default value specified, it may omit the `init(){:tact}` function declaration altogether. That's because unless explicitly declared, the empty `init(){:tact}` function is present by default in all contracts.
+
+The following is an example of a valid empty contract:
+
+```tact
+contract IamEmptyAndIknowIt {}
+```
+
 <Callout>
 
   To obtain initial state of the target contract in [internal functions](#internal-functions), [receivers](#receiver-functions) or [getters](#getter-functions) use [`initOf{:tact}`](/book/expressions#initof) expression.
@@ -201,7 +206,6 @@ They can only be called from [receivers](#receiver-functions), [getters](#getter
 ```tact
 contract Functions {
     val: Int = 0;
-    init() {}
 
     // this contract method can only be called from within this contract and access its variables
     fun onlyZeros() {

pages/book/exit-codes.mdx

@@ -175,9 +175,7 @@ Example:
 1. First, define an empty contract like below:
 
 ```tact
-contract Fireworks {
-    init() {}
-}
+contract Fireworks {}
 ```
 
 2. Then, send a message to this contract. Because no suitable operation is found, you will get this exit code.
@@ -301,4 +299,4 @@ Example:
 // fun newAddress(chain: Int, hash: Int): Address;
 // creates a new address from chain and hash values.
 let zeroAddress: Address = newAddress(-1, 0); // masterchain zero address
-```
+```

pages/book/expressions.mdx

@@ -208,7 +208,6 @@ Anywhere in the function body, a global [static function](/book/functions#global
 
 ```tact
 contract ExampleContract {
-    init() {}
     receive() {
         now(); // now() is a static function of stdlib
         let expiration: Int = now() + 1000; // operation and variable declaration

pages/book/maps.mdx

@@ -289,9 +289,6 @@ contract Example {
     arr: map<Int, Int>; // "array" of String values as a map
     arrLength: Int = 0;    // length of the "array", defaults to 0
 
-    // Constructor (initialization) function of the contract
-    init() {}
-
     // Internal function for pushing an item to the end of the "array"
     fun arrPush(item: String) {
         if (self.arrLength >= self.MaxMapSize) {

pages/book/types.mdx

@@ -116,8 +116,6 @@ contract HelloWorld {
     counter: Int;
 
     // Constructor function init(), where all the variables are initialized
-    // Note, that an empty init function won't be needed starting with Tact 1.3.0,
-    // see: https://github.com/tact-lang/tact/pull/167
     init() {
         self.counter = 0;
     }