Generic/ConstructorName: more defensive coding

This sniff could throw the following fatal errors when it encountered parse errors/during live coding:

```
Fatal error: Uncaught PHP_CodeSniffer\Exceptions\RuntimeException: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in path/to/PHPCS/src/Standards/Generic/Sniffs/NamingConventions/ConstructorNameSniff.php on line 83 in path/to/PHPCS/src/Runner.php on line 624

Fatal error: Uncaught PHP_CodeSniffer\Exceptions\RuntimeException: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in path/to/PHPCS/src/Standards/Generic/Sniffs/NamingConventions/ConstructorNameSniff.php on line 167 in path/to/PHPCS/src/Runner.php on line 624
```

Fixed now by adding more defensive coding.

Includes tests safeguarding the fix.

Author: jrfnl

src/Standards/Generic/Sniffs/NamingConventions/ConstructorNameSniff.php

@@ -78,8 +78,13 @@ protected function processTokenWithinScope(File $phpcsFile, $stackPtr, $currScop
             $this->currentClass = $className;
         }
 
-        $methodName = strtolower($phpcsFile->getDeclarationName($stackPtr));
+        $methodName = $phpcsFile->getDeclarationName($stackPtr);
+        if ($methodName === null) {
+            // Live coding or parse error. Bow out.
+            return;
+        }
 
+        $methodName = strtolower($methodName);
         if ($methodName === $className) {
             if (in_array('__construct', $this->functionList, true) === false) {
                 $error = 'PHP4 style constructors are not allowed; use "__construct()" instead';
@@ -164,7 +169,13 @@ protected function loadFunctionNamesInScope(File $phpcsFile, $currScope)
                 continue;
             }
 
-            $this->functionList[] = trim(strtolower($phpcsFile->getDeclarationName($i)));
+            $methodName = $phpcsFile->getDeclarationName($i);
+            if ($methodName === null) {
+                // Live coding or parse error. Ignore.
+                continue;
+            }
+
+            $this->functionList[] = trim(strtolower($methodName));
 
             if (isset($tokens[$i]['scope_closer']) !== false) {
                 // Skip past nested functions and such.

src/Standards/Generic/Tests/NamingConventions/ConstructorNameUnitTest.2.inc

@@ -0,0 +1,9 @@
+<?php
+
+// Intentional parse error (missing method name).
+// This should be the only test in this file.
+// Testing that the sniff is *not* triggered.
+
+class My_Class {
+    public function {}
+}

src/Standards/Generic/Tests/NamingConventions/ConstructorNameUnitTest.3.inc

@@ -0,0 +1,7 @@
+<?php
+
+// Intentional parse error (missing function name).
+// This should be the only test in this file.
+// Testing that the sniff is *not* triggered.
+
+function