ANTLR4.7: rule XXX contains a closure with at least one alternative that can match an empty string'

I am trying to create a grammar to match content like below:

(For a simple grammar to repro this issue please see ADD 1)

[Defines]
  INF_VERSION                    = 0x00010005
  BASE_NAME                      = WebServer
  FILE_GUID                      = 99E87DCF-6162-40c5-9FA1-32111F5197F7
  MODULE_TYPE                    = SEC
  UEFI_SPECIFICATION_VERSION     = 0x00010005

The UEFI_SPECIFICATION_VERSION = 0x00010005 part is optional.

(for brevity, I omitted some of the grammar).

My grammar 1 looks like this:

defines : '[Defines]'
         define_statement+
         ;

define_statement  : 'INF_VERSION' EQ SpecVersion_VersionVal 
                  | 'BASE_NAME' EQ BaseName
                  | 'FILE_GUID' EQ RegistryFormatGUID
                  | 'MODULE_TYPE' EQ Edk2ModuleType
                  | ('UEFI_SPECIFICATION_VERSION' EQ SpecVersion_VersionVal)?
                  ;

ANTLR 4.7 reports this error:

message: 'rule defines contains a closure with at least one alternative that can match an empty string'

But if I changed grammar like this:

defines : '[Defines]'
         define_statement+
         | ('UEFI_SPECIFICATION_VERSION' EQ SpecVersion_VersionVal)? // <<< HERE
         ;

define_statement  : 'INF_VERSION' EQ SpecVersion_VersionVal
                  | 'BASE_NAME' EQ BaseName
                  | 'FILE_GUID' EQ RegistryFormatGUID
                  | 'MODULE_TYPE' EQ Edk2ModuleType

The error is gone.

My question is, what does the closure mean? Which part is the closure? The define_statement?

After I move the potentially empty alternative, the defines rule can alternate between '[Defines]' define_statement+ and ('UEFI_SPECIFICATION_VERSION' EQ SpecVersion_VersionVal)?, which means defines can still match empty string. How could the error be gone?

ADD 1

To make things more clear, I repro this error with a simplified grammar:

grammar test;

rule : alternate+; // <<<<< HERE
alternate : '1'?;

If I use + or * at HERE, ANTLR will report an error:

'rule rule contains a closure with at least one alternative that can match an empty string'

If I use ? at HERE, ANTLR will report a warning:

'rule rule contains an optional block with at least one alternative that can match an empty string'

I am still not sure why.

ADD 2

Each of the alternate WILL be a child node of rule, so if alternate can be empty string, then it is logically possible to lead to endless child nodes for rule. So I guess this may explain why ANTLR forbids me to do that with alternate+ or alternate*. But if it is with alternate?, at most there will be one child node. It's only a performance issue. So ANTLR just generate a warning.