sync_diff_inspector: prevent float/double 0 turned into NULL

[kennytm]

What problem does this PR solve?

Issue Number: close #841

What is changed and how it works?

Replace the computation $(\log_{10} |x|)$ by $(\log_{10} \max\{|x|, \epsilon\})$ so that even when x = 0 we won't have log10(0) producing a NULL.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

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