Fix compiling of PHP-injected variables with false, null or empty string

Less.php 5.1.0 could no longer compile variables injected via
Less_Parser->ModifyVars if they have an empty value.

This bug was introduced in Ie8d5da3ed47b817bc9dd79070488509a7aa2feb2.
In Less.js v3 and above, this remains an unsolved bug.

In Less.js < 3.0 and Less.php < 5.1, the following could compile:

```
@foo : ;

div{
    color: @foo;
}
```

Currently, the block above returns an error. We decided to work around this by using "~" with an empty string to serialize an empty value. We caught this in CI while drafting a mediawiki/vendor patch for the
Less.php 5.1.0 upgrade at https://gerrit.wikimedia.org/r/1060459.

Change-Id: I1bafdbc527a2fef62a0eb7ed36740b38726cb1c4

lib/Less/Parser.php

@@ -3222,6 +3222,9 @@ public static function serializeVars( $vars ) {
 		$s = '';
 
 		foreach ( $vars as $name => $value ) {
+			if ( strval( $value ) === "" ) {
+				$value = '~""';
+			}
 			$s .= ( ( $name[0] === '@' ) ? '' : '@' ) . $name . ': ' . $value . ( ( substr( $value, -1 ) === ';' ) ? '' : ';' );
 		}
 

test/phpunit/ParserTest.php

@@ -277,12 +277,36 @@ public function testOptionRootpath() {
 		$this->assertEquals( $expected, $css );
 	}
 
+	public function testModifyVars() {
+		$lessCode = <<<CSS
+			div{
+				border-width: @border-width;
+				line-height: unit(@line-height, px);
+				border-radius: @border-radius;
+				margin: @margin;
+			}
+			CSS;
+		$expected = <<<CSS
+			div {
+			  border-width: ;
+			  line-height: 1px;
+			  border-radius: ;
+			  margin: ;
+			}
+			CSS;
+		$parser = new Less_Parser;
+		$parser->ModifyVars( [ 'border-width' => false, 'line-height' => true, 'border-radius' => null, 'margin' => '' ] );
+		$parser->parse( $lessCode );
+		$css = trim( $parser->getCss() );
+		$this->assertSame( $expected, $css, 'static callback' );
+	}
+
 	public function testOptionFunctions() {
 		$lessCode = <<<CSS
 			#test {
 			  border-width: add(7, 6);
 			}
-		CSS;
+			CSS;
 		$expected = <<<CSS
 			#test {
 			  border-width: 13;