Pandas if-else "join" function returns ValueError: The truth value of a Series is ambiguous

Below is the example of my dataset:

╔═══╦════════════╦═══════════════╗
║   ║ col_1      ║ col_2         ║
╠═══╬════════════╬═══════════════╣
║ 1 ║ 106        ║ I am Alex.    ║
║ 2 ║ 106        ║ I'm a student ║
║ 3 ║ 106        ║ I like apple  ║    
║ 4 ║ 1786       ║ Dog is a pet  ║
║ 5 ║ 1786       ║ Jack is my pet║
╚═══╩════════════╩═══════════════╝

and I would like to first groupby "col_1" and then join the string in "col_2" with the if-else condition of finding the last character in the string whether it is ended with "."

If it is ended with a fullstop, join the next string of the same group with " ".join (join them with a space). Else, join them with a fullstop.

End result will look something like this:

╔═══╦════════════╦══════════════════════════════════════════╗
║   ║ col_1      ║ col_2                                    ║
╠═══╬════════════╬══════════════════════════════════════════╣
║ 1 ║ 106        ║ I am Alex. I'm a student. I like apple   ║
║ 2 ║ 1786       ║ Dog is a pet. Jack is my pet             ║
╚═══╩════════════╩══════════════════════════════════════════╝

My code is stated as below:

new_df = df.groupby(['col_1'])['col_2'].apply(lambda x: ' '.join(x) if x[-1:] == '.' else '. '.join(x)).reset_index()

However I got this error instead:

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

Your help is much appreciated!