fix(chatform): Fix invalid access of empty container

[sphaerophoria]

This partially fixes #5832 in that we no long segfault. There is a
deeper underlying problem that the history still relies on undefined
behavior of GROUP BY causing us to get an empty container in some cases.

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This change is 

[sudden6]

Reviewable status: 0 of 1 LGTMs obtained

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src/widget/form/genericchatform.cpp, line 689 at r1 (raw file):

    auto end = ChatLogIdx(0);
    if (time.isNull()) {
        end = messages.begin()->first;

Isn't this also a potential crash if messages is empty?

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src/widget/form/genericchatform.cpp, line 720 at r1 (raw file):

    auto begin = ChatLogIdx(0);
    if (time.isNull()) {
        begin = messages.rbegin()->first;

same

[anthonybilinski]

rebased to v1.17-dev tip. It does look like there are a couple unguarded usages of messages's first item.