ConfigurationPropertyName incorrectly accepts an opening square bracket at the end of a name

[zhoubiao188]

ConfigurationPropertyName rejects names with a bracket that is left unterminated.
ElementsParser.parse records this by marking the element it is currently building as
NON_UNIFORM, and elementsOf rejects any name that contains a NON_UNIFORM element:

if (openBracketCount != 0) {
    type = ElementType.NON_UNIFORM;
}
add(start, length, type, valueProcessor);

When the unbalanced bracket is the last character of the name, the element being
built is empty and add discards empty elements, taking the NON_UNIFORM marker with
it. Such names are accepted:

ConfigurationPropertyName.of("foo[");       // no exception
ConfigurationPropertyName.isValid("foo[");  // true

while the equivalent names with content are correctly rejected:

ConfigurationPropertyName.of("[bar");    // InvalidConfigurationPropertyNameException
ConfigurationPropertyName.of("foo[bar"); // InvalidConfigurationPropertyNameException

The accepted name is also self-inconsistent. Its only element is foo, but toString()
returns foo[ (the source is used as a shortcut), so it is not equal to the name foo
and it does not round-trip through ConfigurationPropertyName.of.

With this change the unbalanced bracket is included in the final element so that it is
not dropped as an empty element, and the name is reported as invalid — the same outcome
as for foo[bar:

ConfigurationPropertyName.of("foo[");       // InvalidConfigurationPropertyNameException
ConfigurationPropertyName.isValid("foo[");  // false

InvalidConfigurationPropertyNameException.getInvalidCharacters() reports [, which
matches the existing behaviour for [bar.

Regression tests are included for foo[, for the name consisting of just [, and for
isValid. Without the fix these fail; with it they pass. The rest of the
org.springframework.boot.context.properties test tree is unaffected.