[LANG-1804] Fix CharSet#getInstance returns null instead of EMPTY when input setStrs is null

[IcoreE]

// source code
public static CharSet getInstance(final String... setStrs) {
    if (setStrs == null) {
        return null; // error
    }
    if (setStrs.length == 1) {
        final CharSet common = COMMON.get(setStrs[0]);
        if (common != null) {
            return common;
        }
    }
    return new CharSet(setStrs);
} 

The getInstance method of CharSet currently returns null when the input parameter setStrs is null. This behavior contradicts the documentation, which states that a null input should return the EMPTY constant (an empty character set). Returning null can lead to NullPointerException in calling code that expects a valid CharSet instance even for empty inputs.

The Javadoc and source code are as follows：

// Test NullPointerException
    @Test
    void testGetInstance_Null_Fixed_ReturnsEmpty_NoNPE() {
        CharSet charSet = CharSet.getInstance(null);
        assertThrows(NullPointerException.class,()->charSet.contains('['));
    } 

[garydgregory]

@IcoreE If you don't run mvn by itself before pushing, you'll never catch build errors like the ones that just occurred (see the instructions in the PR template).

[garydgregory]

Jira: https://issues.apache.org/jira/browse/LANG-1804