[php-src] Issue #20632: Use better error message for non-numeric strings in implicit contexts

Issue: https://github.com/php/php-src/issues/20632
Author: IMSoP

### Description

When a "non-numeric" string is coerced to an integer or float, PHP raises an error rather than proceeding with the implicit cast. This is a good thing.

However, the error message implies that _any_ string would be invalid, rather than that the _content_ of the string is the problem.

Additionally, a common cause in my experience is that the string is _empty_, for instance, missing from some input data. It would be helpful for debugging to highlight when this is the case.

## Proposal

When coercion to a numeric, float, or int context fails:

* If the string being coerced is empty, use "empty string" as a pseudo-type in the error message
* Otherwise, use "non-numeric string" as a pseudo-type in the error message

## Current Behaviour

Mathematical operators:

```php
echo 1 + "1";
// 2
echo 1 + "hello";
// TypeError: Unsupported operand types: int + string
echo 1 + "";
// TypeError: Unsupported operand types: int + string
```

Function calls in coercive-call mode:

```php
declare(strict_types=0);

function foo(int $a) { echo $a; }

foo("1");
// 1
foo("a");
// TypeError: foo(): Argument #1 ($a) must be of type int, string given
foo("");
// TypeError: foo(): Argument #1 ($a) must be of type int, string given
```

## Proposed behaviour

Mathematical operators:

```php
echo 1 + "1";
// 2
echo 1 + "hello";
// TypeError: Unsupported operand types: int + non-numeric string
echo 1 + "";
// TypeError: Unsupported operand types: int + empty string
```

Function calls in coercive-call mode:

```php
declare(strict_types=0);

function foo(int $a) { echo $a; }

foo("1");
// 1
foo("a");
// TypeError: foo(): Argument #1 ($a) must be of type int, non-numeric string given
foo("");
// TypeError: foo(): Argument #1 ($a) must be of type int, empty string given
```