regex for blank string

You can exclude matching the ] instead in the capture group, and if you also want to match the value on the last line, you can assert the end of the string $ instead of matching a mandatory newline with \n

Note that \s can match a newline and also the negated character class [^]]* can match a newline

]:\s+([^]]*)$

Regex demo | Python demo

import re

regex = r"]:\s+([^]]*)$"

s = ("(2021-06-29T10:53:42.647Z) [Denis]: hi\n"
    "(2021-06-29T10:54:53.693Z) [Nicholas]: TA FOR SHOWING\n"
    "(2021-06-29T11:58:29.053Z) [Nicholas]: how are you bane \n"
    "(2021-06-29T11:58:29.053Z) [Nicholas]: \n"
    "(2021-06-29T11:58:29.053Z) [Nicholas]: #END_REMOTE#\n"
    "(2021-06-30T08:07:42.029Z) [Denis]: VAL 01JUL2021\n"
    "(2021-06-30T08:07:42.029Z) [Denis]: ##ENDED AT 08:07 GMT##")

print(re.findall(regex, s, re.MULTILINE))

Output

['hi', 'TA FOR SHOWING', 'how are you bane ', '', '#END_REMOTE#', 'VAL 01JUL2021', '##ENDED AT 08:07 GMT##'] 

If you don't want to cross lines:

]:[^\S\n]+([^]\n]*)$

Regex demo

You could identify all after the colon into an array from capture group 1.

re.findall(r'(?m):[ \t]+(.*?)[ \t]*$',s) 

then loop the array assigning a space to all empty elements.

>>> import re
>>>
>>> s= """
... (2021-06-29T10:53:42.647Z) [Denis]: hi
... (2021-06-29T10:54:53.693Z) [Nicholas]: TA FOR SHOWING
... (2021-06-29T11:58:29.053Z) [Nicholas]: how are you bane
... (2021-06-29T11:58:29.053Z) [Nicholas]:
... (2021-06-29T11:58:29.053Z) [Nicholas]: #END_REMOTE#
... (2021-06-30T08:07:42.029Z) [Denis]: VAL 01JUL2021
... (2021-06-30T08:07:42.029Z) [Denis]: ##ENDED AT 08:07 GMT##
... """
>>>
>>> talk = [re.sub('^$', ' ', w) for w in re.findall(r'(?m):[ \t]+(.*?)[ \t]*$',s)]
>>> print(talk)
['hi', 'TA FOR SHOWING', 'how are you bane', ' ', '#END_REMOTE#', 'VAL 01JUL2021', '##ENDED AT 08:07 GMT##']

With your shown samples please try following regex.

^\(\d{4}-\d{2}-\d{2}T(?:\d{2}:){2}\d{2}\.\d{3}Z\)\s+\[[^]]*\]:\s+([^)]*)$

Online demo for above regex

Explanation: Adding detailed explanation for above.

^\(\d{4}-\d{2}-\d{2}  ##Matching from starting of line ( followed by 4 digits-2 digits- 2 digits here.
T(?:\d{2}:){2}        ##Matching T followed by a non-capturing group which is matching 2 digits followed by colon 2 times.
\d{2}\.\d{3}Z\)\s+    ##Matching 2 digits followed by dot followed by 3 digits Z and ) followed by space(s).
\[[^]]*\]:\s+         ##Matching literal [ till first occurrence of ] followed by ] colon and space(s).
([^)]*)$              ##Creating 1st capturing group which has everything till next occurrence of `)`.

With Python3x:

import re
regex = r"^\(\d{4}-\d{2}-\d{2}T(?:\d{2}:){2}\d{2}\.\d{3}Z\)\s+\[[^]]*\]:\s+([^)]*)$"
varVal = ("(2021-06-29T10:53:42.647Z) [Denis]: hi\n"
    "(2021-06-29T10:54:53.693Z) [Nicholas]: TA FOR SHOWING\n"
    "(2021-06-29T11:58:29.053Z) [Nicholas]: how are you bane \n"
    "(2021-06-29T11:58:29.053Z) [Nicholas]: \n"
    "(2021-06-29T11:58:29.053Z) [Nicholas]: #END_REMOTE#\n"
    "(2021-06-30T08:07:42.029Z) [Denis]: VAL 01JUL2021\n"
    "(2021-06-30T08:07:42.029Z) [Denis]: ##ENDED AT 08:07 GMT##")

print(re.findall(regex, varVal, re.MULTILINE))

Output will be as follows with samples shown by OP:

['hi', 'TA FOR SHOWING', 'how are you bane ', '', '#END_REMOTE#', 'VAL 01JUL2021', '##ENDED AT 08:07 GMT##']