﴿ وَأَصْبَحَ فُؤَادُ أُمِّ مُوسَىٰ فَٰرِغًا ۖ إِن كَادَتْ لَتُبْدِى بِهِۦ لَوْلَآ أَن رَّبَطْنَا عَلَىٰ قَلْبِهَا لِتَكُونَ مِنَ ٱلْمُؤْمِنِينَ ﴾

“On the morrow, however, an aching void grew up in the heart of the mother of Moses, and she would indeed have disclosed all about him had We not endowed her heart with enough strength to keep alive her faith [in Our promise].”

And the heart of Moses’s mother when she found out that they had picked him up became empty of everything other than him. Indeed in softened from the hardened form its subject omitted in other words understand it as innaha she was about to expose him that is as being her son had We not fortified her heart with patience that is We made it at peace that she might be of the believers of those who have faith in God’s promise the response to the conditional lawlā ‘had … not’ is given by the preceding statement.